Rest assured, your theme’s style.css is not missing (most likely). However, maybe you’ve just uploaded your theme and you received the following error:

The following themes are installed but incomplete. Themes must have a stylesheet and a template. Stylesheet is missing.

Usually this means something happened to the theme during the upload or install. It could be that you’re trying to upload or install the wrong theme files. The proper files will be titled themename.zip, and will contain only the theme files. Sometimes your download may contain both the theme zip file and the help files. To check, you can simply unzip your download just to be sure.