what are the css files with question marks - like modules/system/system.theme.css?pywps6u? I am new here, starting to find my way around a Drupal site I inherited. I am trying to work out if I can safely ignore all the @import css files with question marks like system.theme.css?pywps6u . I notice that the appended string of letters changes every so often, and I assume this is resulting from the caching.

What's going on here and can I safely ignore these files and just edit the normal files with standard filenames like MyStyleSheet.css?
I assume this is resulting from the caching


You are correct. It is for caching; especially to prevent browsers from thinking they have already downloaded a certain file.

If the file was named system.theme.css, and your browser already downloaded that file several days ago, it won't download it again. This results in an outdated site. The suffix changes every time the CSS files change and forces your browser to download the file again.

You can safely edit the 'regular' CSS files on the filesystem. Take care though that is discouraged to edit system and module files, as they will be overridden during updates. Instead create a custom module or theme to implement the changes you need.