Hi there,
Every theme is different, so there’s no real place to learn theme specific CSS that will apply to all themes.
Your best bet is to learn to use the Inspect Element functionality of your browser.
If you’re using Chrome, just right click and select “Inspect”.
If you’re using Firefox, install the Firebug extension.
These tools let you comb through the HTML of the theme and shows you the CSS that applies to each element.
It can be super helpful in understanding the workings of the theme and overwriting styles etc..
Hope this helps ๐