Try using the Web Developer in your browser (usually activated by F12, or a right-click and "Inspect Element" or something like that). This will let you click on elements on the page (like a tag) and see what class they belong to. It usually also shows you the CSS for that class, and any others it inherits, so you can see what causes what styles.
That's a good place to start. Changing a theme is pretty intensive work, so don't give up. Eventually Flarum will have an easier way to do it, but for now you either have to overwrite the LESS files yourself or use the Custom CSS box in the admin panel.
