@Salamander23 : with all due respect for your far and away superior CSS knowledge than mine, and agreeing that a theme’s CSS file can be modified, I don’t think any modifications should be made in it, contrary to what you said to @Cajun.
The reason is that if the theme gets updated, all those modification will be lost as the updated theme’s CSS file overwrites the previous (= modified) one.
The best way to make changes to a them is to use snippets, as they are not affected by theme updates. Furthermore, they override the theme’s CSS.
My apologies if I sound pedantic, I don’t mean to be.