If you put it in styles.css then it is included in every page.
If you want to target just the single page, then take a look at the classes applied to the body tag on each page and that will allow you to target the CSS selectors quite specifically.
e.g. this page (on Moodle.org) has the body classes:
- path-mod
- path-mod-forum
If you want to be even more specific, it has the body id of:
- page-mod-forum-post
And if you are wanting to be ridiculously specific, you can even target the course, category, context and course module ids, via the classes.