This is one of Arty's (Artodia
) styles - in certain aspects they are a little "special":
(see => https://www.phpbb.com/customise/db/styl ... pic/141916 )
CSS files have been changed to Sass, a very powerful language for generating stylesheets. Style loads only 1 CSS file instead of multiple CSS files, reducing page loading speed.
In standard prosilver and its direct derivates stylesheet.css contains the list of the corresponding style's stylesheets in the order they are being imported (= loaded) so there is nothing one should or could edit in this file (besides adding further, self-created stylesheets if needed).
All editing is done in the individual stylesheets.
In terms of technical correctness this may be somehow over-simplified:
Arty's style use SASS, providing a CSS compatible syntax with a lot more features than standard CSS. Compiling "translates" this into "regular" CSS and "writes" all of the resulting CSS code into one single file => stylesheet.css.
Each compiler run will re-create this file from scratch, whiping out any earlier versions of it.
That's why you never edit stylesheet.css - you edit either individual .scss files or, in certain cases, the ones I mentioned in an earlier post.
(again see => https://www.phpbb.com/customise/db/styl ... pic/141916 )
What matters is in this style you should never edit stylesheet.css!
You should follow Arty's directions to avoid running into problems in the process of further individualizing this style sooner or later.