Why it happens

You uploaded the wrong file. Theme marketplaces often give you a bundle with the real theme zip, documentation and licence files inside. WordPress unpacks the bundle and finds no style.css. This is the usual cause.
The theme folder is one level too deep, such as download/my-theme/style.css. WordPress looks inside the single top folder, here download/, and finds no style.css there.
The zip has two or more folders at the top, such as the theme and a docs folder. WordPress then looks at the very top of the zip, not inside either folder.
The file is not a theme. A plugin zip, or a design for another platform, has no style.css either.
The file is named differently. Style.css, styles.css or main.css do not count. On Linux servers the name must be exactly style.css.
For a theme that is already installed: something deleted or renamed its style.css, usually a failed update, a cleanup script or a manual change over FTP. The page then shows Stylesheet is missing. in the Broken Themes list.

How to fix it

Try the fixes in this order. Each one is independent: stop as soon as the error is gone.

FIX 01

Look inside the zip first

On your computer, run unzip -l your-theme.zip, or open the zip. It lists the files without unpacking them.
A good theme zip shows one folder, and style.css directly inside it: my-theme/style.css.
If you see another .zip, or two folders before style.css, or no style.css at all, use the matching fix below.

FIX 02

Upload the inner theme zip

Unzip the bundle you downloaded. Look for a smaller .zip named like the theme. Vendors often put it in a folder called installable, theme or Theme Files.
In Appearance → Themes → Add Theme → Upload Theme, choose that inner zip and click Install Now.
I tested a zip that held only another zip. WordPress gave the message in this page. The inner zip installed.

FIX 03

Zip the theme folder again

Unzip the file. Open the folders until you reach the one that has style.css in it. That is the theme folder.
Zip that folder, not the one above it. On a Mac, right-click it and choose Compress. On Windows, right-click it and choose Send to → Compressed (zipped) folder. In a terminal: zip -r my-theme.zip my-theme.
Check it with unzip -l my-theme.zip. my-theme/style.css should be listed.
If the zip also had a docs or licence folder, leave it out of the new zip. I tested this: with the extra folder the install failed, without it the theme installed.
A zip with style.css and index.php directly at the top, with no folder, also installed. WordPress made a folder named loose- plus random letters. Use a named folder instead.

FIX 04

Use the other uploader if it is a plugin

Plugin zips go in Plugins → Add Plugin → Upload Plugin. A plugin zip sent to the theme uploader gives this message. I tested it.
If you are not sure what the file is, run unzip -l and look for a PHP file that starts with a Plugin Name: comment. That is a plugin.

FIX 05

Rename the file to exactly style.css

In the unzipped folder, check that the file is called style.css, all lower case. A theme with Style.css was refused on my Linux lab site; a Linux server tells Style.css and style.css apart.
If style.css is deeper, such as my-theme/assets/style.css, move it to the top of my-theme/. WordPress does not look in sub-folders. I tested that too.
Zip the folder again and upload it.

FIX 06

Put a missing style.css back on an installed theme

Open Appearance → Themes. If the Broken Themes list at the bottom says Stylesheet is missing., the theme folder is still there, but style.css is not.
Restore it from a backup, or install the theme again. If the theme came from wordpress.org, Appearance → Themes → Add Theme installs a fresh copy.
If the theme is not one you need, click Delete in that row.
I tested the restore: I put style.css back, and WordPress no longer listed the theme as broken.

FIX 07

Add the Template line to a child theme

A row that reads Template is missing. means a theme has a style.css but no index.php and no templates/index.html. If it is meant to be a child theme, open its style.css and add a line Template: parent-folder-name in the top comment.
The value is the folder name of the parent theme, for example twentytwentyfive, not its display name. The parent must be installed.
If the row reads The parent theme is missing. Please install the "…" parent theme., install the parent. I tested both: the first row went away with the Template line, the second when the named parent was installed.
A theme that is meant to stand alone needs a template file: index.php for a classic theme, or templates/index.html for a block theme.

The message means WordPress unpacked your zip and found no file called style.css in the place it looks. Nearly always you uploaded the wrong zip, or the theme sits one folder too deep. Look inside the zip with unzip -l, find the folder that holds style.css, and upload that.

What you see#

On Appearance → Themes → Add Theme → Upload Theme, the page that unpacks the zip stops here:

The WordPress admin page titled Installing theme from uploaded file: wrapped.zip. Below it: Unpacking the package, Installing the theme, The package could not be installed. The theme is missing the style.css stylesheet. Theme installation failed. — Uploading a zip that holds the theme one folder too deep, on WordPress 7.1.4.

WP-CLI shows the same text, with the code tags left in:

Terminal

wp theme install wrapped.zip

Output

Theme installation failed.Warning: The package could not be installed. "The theme is missing the <code>style.css</code> stylesheet."Warning: The '/tmp/zips/wrapped.zip' theme could not be found.Error: No themes installed.

The same words also have a second life. If a theme that is already on the site loses its style.css, Appearance → Themes shows a Broken Themes list with different wording. See "The Broken Themes list" below.

Where WordPress looks#

After unpacking, Theme_Upgrader::check_package() picks one folder to look in. If the zip holds exactly one folder at the top, it looks inside that folder. In every other case it looks at the top of the unpacked zip. It does not look any deeper.

Four boxes. 1: style.css in the top folder lab-good installs. 2: the theme one folder too deep, in dl/lab-good, fails because WordPress looks in dl. 3: a zip inside the zip fails. 4: two top-level folders fail because WordPress looks at the top. — Four zip layouts and what WordPress does with each. Layouts 2 to 4 gave the message in this page on WordPress 7.1.4.

This is why the file listing matters. unzip -l on the failing zip from the screenshot:

Terminal

unzip -l wrapped.zip

Output

Archive:  wrapped.zip  Length      Date    Time    Name---------  ---------- -----   ----        0  10-30-2026 18:28   dl/        0  10-30-2026 18:28   dl/lab-good/       15  10-30-2026 18:28   dl/lab-good/index.php       40  10-30-2026 18:28   dl/lab-good/style.css---------                     -------       55                     4 files

The only top folder is dl/, and dl/ has no style.css. Zipping dl/lab-good as lab-fixed.zip gave a listing starting with lab-fixed/, and it installed with "Theme installed successfully."

The check runs in this order, and each one stops the install with its own message:

Check	What it needs	Message
1	`style.css` in the folder it picked	The theme is missing the style.css stylesheet.
2	A `Theme Name:` line in it	The style.css stylesheet does not contain a valid theme header.
3	`index.php`, `templates/index.html` or `block-templates/index.html`, unless it has a `Template:` line	Template is missing. Standalone themes need to have a templates/index.html or index.php template file. Child themes need to have a Template header in the style.css stylesheet.
4	The site meets `Requires PHP:` and `Requires at least:`	The PHP version on your server is …, however the uploaded theme requires ….

Checks 2 to 4 start with the same words as this page, "The package could not be installed.", so people often search for the whole sentence. I produced 2 and 3 on WordPress 7.1.4 by uploading a style.css that was only body{} and a theme with a style.css and nothing else. Check 4 I read in the code but did not run.

Every zip I tried#

All run on WordPress 7.1.4 with wp theme install:

Zip contents	Result
`lab-good/style.css`, `lab-good/index.php`	Installed
`style.css` and `index.php` at the top, no folder	Installed, in a folder named `loose-` plus random letters
Block theme: `style.css`, `templates/index.html`	Installed
Child theme: `style.css` with `Template: lab-good` and nothing else	Installed (it said the parent was not found, and still installed)
`dl/lab-good/style.css`	The theme is missing the style.css stylesheet.
`lab-good/` and `docs/` at the top	The theme is missing the style.css stylesheet.
A zip holding only `lab-good.zip`	The theme is missing the style.css stylesheet.
A plugin zip	The theme is missing the style.css stylesheet.
`Style.css` (capital S)	The theme is missing the style.css stylesheet.
`lab-sub/assets/style.css`	The theme is missing the style.css stylesheet.
Block theme files with `theme.json` and `templates/index.html` but no `style.css`	The theme is missing the style.css stylesheet.
`style.css` with no `Theme Name:`	The style.css stylesheet does not contain a valid theme header.
`style.css` only, no `Template:` line	Template is missing. …
A file that is not a zip (HTML saved as `.zip`)	The package could not be installed. "PCLZIP_ERR_BAD_FORMAT (-10) : Unable to find End of Central Dir Record signature"

The block theme row matters. Block themes need no index.php, but they still need style.css. The header in it is how WordPress learns the theme's name.

The Broken Themes list#

A theme that is already installed can break too. The list at the bottom of Appearance → Themes says "The following themes are installed but incomplete." I produced all three rows with the messages WordPress 7.1.4 uses:

The Broken Themes table on the Themes page. lab-active: Stylesheet is missing. Lab Child: The parent theme is missing. Please install the not-installed parent theme. Lab Child No Template: Template is missing. Standalone themes need to have a templates/index.html or index.php template file. Child themes need to have a Template header in the style.css stylesheet. Each row has a Delete link. — The Broken Themes list on a lab site: one theme whose style.css was renamed, one child theme with a parent that is not installed, one style.css with no Template line.

Row text	What WordPress found	Fix
Stylesheet is missing.	The theme folder, but no `style.css` in it	Restore `style.css`, or reinstall the theme
Template is missing. …	A `style.css`, but no `index.php` or `templates/index.html`, and no `Template:` line	Add the `Template:` line (child theme), or add the template file
The parent theme is missing. Please install the "…" parent theme.	A child theme whose `Template:` names a folder that is not there	Install the parent with exactly that folder name

Two things I found that other pages get wrong.

A folder with no style.css does not show up at all, unless it is the active theme. WordPress builds the theme list by looking for style.css. I renamed style.css in a theme that was not active: it vanished from wp theme list and from the Themes screen, with no Broken Themes row. A "Stylesheet is missing." row appears only for the theme named in the site's settings.

A broken active theme is replaced on the next visit to Appearance → Themes. In my test I renamed style.css in the active theme. The front end kept answering with status 200, because it was a block theme and its styles came from theme.json. When I opened Appearance → Themes, WordPress showed "The active theme is broken. Reverting to the default theme." and the site's stylesheet setting changed to twentytwentyfive. The validate_current_theme() function does this when the active theme has no style.css or no index file. Restoring the file afterwards did not switch the theme back. I activated it again myself. Note that this is a change to a live site made by simply opening an admin page, so fix the file before you click around.

If it keeps happening#

If the zip is right on your computer but still fails, the download is the likely problem: run unzip -t on it, and download it again. A file that is not a zip at all gives the "PCLZIP_ERR_BAD_FORMAT" text from the last table row; see incompatible archive: the package could not be installed. If the zip is right and WordPress then cannot write the files, that is a different message: see could not create directory. If the theme already exists, see destination folder already exists.

Before you replace or delete a theme on a live site, make a backup. The routine is in how to update plugins safely; it works the same for themes.

Sources

Main stylesheet (style.css) - Theme Handbook · style.css is required in the theme's root folder; Theme Name is the header.
Child Themes - Theme Handbook · The Template header must match the parent's folder name.
class-theme-upgrader.php (Theme_Upgrader::check_package) - wordpress-develop · The three install checks: style.css exists, it has a Theme Name, the theme has an index file.
class-wp-theme.php - wordpress-develop · Where Stylesheet is missing., Template is missing. and the parent theme message come from.
validate_current_theme() - wordpress-develop · Why a broken active theme is replaced by the default theme.

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