This is one of Arty's (
Artodia) styles - in certain aspects they are a little "special":
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.
(see => https://www.phpbb.com/customise/db/styl ... pic/141916 )
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.
What matters is in this style you should never edit stylesheet.css!
(again see => https://www.phpbb.com/customise/db/styl ... pic/141916 )
You should follow Arty's directions to avoid running into problems in the process of further individualizing this style sooner or later.