What does 'The theme is missing the style.css stylesheet' mean in WordPress?

WordPress cannot find the theme's style.css, the file whose top comment contains 'Theme Name:'. Usually the uploaded zip is a marketplace package that holds the real theme zip, or the theme sits in a folder inside another folder.

Why does WordPress say a child theme's parent theme is missing?

A child theme's style.css names its parent in a 'Template:' line. Install the parent theme with that exact folder name first, then activate the child theme.

More fixes

WordPress theme install error: "The theme is missing the style.css stylesheet"

Q: How do I fix 'The package could not be installed. The theme is missing the style.css stylesheet'?

Open the zip on your computer. If it contains another zip named after the theme, upload that inner zip instead. If it holds a folder inside a folder, re-zip so style.css sits directly inside one folder. If it contains a PHP file with 'Plugin Name:', it is a plugin and installs under Plugins.

You upload a theme zip and WordPress stops with "The package could not be installed. The theme is missing the style.css stylesheet." Or a theme shows up under Appearance, Themes as "Broken Themes" with "Stylesheet is missing." The theme is usually fine. WordPress just cannot find its style.css where it expects it. Here is how to tell which case you have, and the fix for each.

What the owner sees

harborbakery.example/wp-admin/update.php?action=upload-theme

What the checks found

Find out which case it is (five minutes)

Open the zip on your computer. A good theme zip holds one folder (for example harbor/) and directly inside it a style.css whose top comment starts with Theme Name:. If the zip holds another zip, or a folder inside a folder, that is the fault.
Is it a "full package"? Themes bought on ThemeForest and similar marketplaces download as a package: documentation, licence, demo content, child theme, and the real theme zip. Uploading the whole package gives exactly this error.
Is it really a theme? A plugin zip (it has a .php file with Plugin Name: and no style.css) belongs under Plugins. A GitHub source download (named like theme-main.zip) may hold build files rather than the theme itself.
"Broken Themes" in the list? Then the files are already on the server. In the host's File Manager or FTP, open wp-content/themes/ and look inside the named folder: style.css should be right there, not one folder further down.

The fixes, by case

Marketplace package: unzip the download on your computer. Upload the inner zip named after the theme (ThemeForest calls it "Installable WordPress file only"). Install the parent theme first, then the child theme zip if you want one.
Folder inside a folder: re-zip so the zip holds one folder with style.css directly inside. On the server, move the inner folder's contents up one level, then reload Appearance, Themes.
It is a plugin: upload it in Plugins, Add New Plugin, Upload Plugin instead.
Child theme shows "The parent theme is missing": the child's style.css has a Template: line naming the parent folder. Install that parent theme (same folder name) and the child works.
Upload fails part-way or the zip is very large: the host's upload limit cut it off, so WordPress unpacked half a theme. Upload the theme folder by FTP or File Manager into wp-content/themes/ instead, then activate it.
You edited style.css: the top comment must still start with /* and contain Theme Name: Something. A file saved with a byte-order mark or a deleted first line hides it. Re-download the theme rather than patching it.

What you see, what it means, what it needs

What you see	What it means	Usual fix
"The theme is missing the style.css stylesheet" on upload	No `style.css` at the top of the zip's folder	Upload the inner theme zip, or re-zip one folder
"Broken Themes: Stylesheet is missing"	The theme folder on the server is nested or incomplete	Move files up one level, or re-upload by FTP
Zip holds `documentation/`, `licensing/` and more zips	A marketplace "full package"	Upload the installable theme zip inside it
"The parent theme is missing"	A child theme whose parent is not installed	Install the parent theme first
Zip has a `.php` with `Plugin Name:`	It is a plugin	Install it under Plugins

What not to do

Do not download a "nulled" copy of a paid theme to get one that installs. Those copies are one of the most common ways WordPress sites get hacked.
Do not create an empty style.css to make the error go away. WordPress will list a theme that has no templates and your site will break when you activate it.
Do not activate a half-uploaded theme on a live site. Check it in Appearance, Customize (Live Preview) first.
Do not keep uploading the same zip. If it failed once, it is packed wrong, and every try can leave another half-made folder in wp-content/themes/.

Most cases are one zip opened and one folder moved. If the theme still will not install, that is what the box below is for. Related: "The plugin does not have a valid header", "Incompatible archive", "Destination folder already exists".