I am kind new to CSS:

.subcat_links {...}
.g-head_links {...}
.category_links {...}
.blabla{...}

How does a webpage know what css to use... How to call, start and end css sections on a page?

oh jeez where to start. The best is to go here

http://www.w3schools.com/css/css_intro.asp

and learn from the beginning. The worst is to learn by bits and pieces (I know from example). I've got style sheets flying all over the place and I'm afraid to delete any of them for fear of finding some small corner of my site still using one of those that I just deleted

In LSQL, if you check your globals, look under css and edit the fonts you want, size color etc for a start and you don't have to do it manually.

I don't know if I led you off topic but good luck.