TiddlyWiki.com can be and, in fact, is a great reference for many a things. Do use it, exhaustively, e.g. searching for StyleSheet.
Other than that, use your browser's developer tools to inspect that element of the page that is of interest to you.
Right click on the element and select "inspect", go to the html element that you want to modify and in the lower right corner there is usually that panel where you can change the element's css. Simply enter your desired style, e.g. border-top: 2px solid purple and see if the outcome pleases you, then try more stuff.
If you're on a tablet, things are going to be harder if not impossible. Then you actually need someone giving you the required code... which is why a
class reference amongst others would be a neat thing.
Best wishes, Tobias.