We have some guides on building themes for Discourse. See: Beginner’s guide to using Discourse themes and Developer’s guide to Discourse themes
There’s not really a guide to HTML/CSS specifically for Discourse, if that’s what you’re looking for.
If you’re somewhat familiar with HTML/CSS the best way to figure out which elements to style would be to use your browser’s inspector (right click, inspect). You can look at mobile view on desktop by adding ?mobile_view=1 to the end of your current URL (go back to desktop with ?mobile_view=0).
If you’re not at all familiar with HTML/CSS, there’s quite a bit to learn. Codecadamy has HTML and CSS tutorials. Mozilla also has HTML and CSS introductions.