This contains all pictures and icons used in the theme. Themes come with pictures for the tabs and eventually for the gradients.
All images linked from header.html and footer.html should use $CFG->httpswwwroot
$meta (all the meta tags, including stylesheets)
The styles.php file is called from header.html and builds the bridge to the CSS files.
You don't need to edit anything much in this file, use config.php instead.
The one variable you may want to change during development, however is $lifetime. Set it to a very small value (like 1 second) to prevent the theme from caching too long when you are making changes.
So, depending on your settings, you may have up to three stylesheets for a theme: