Custom fonts background of category boxes and widget title's in 'Clear' theme

Hey! I’m having a lot of trouble with a couple of things in the Clear theme…

Changing category fonts – It gives me the option of some as a drop down in the theme settings but I don’t understand how to customise that as it hasn’t worked using FTP and editing the CSS. Any ideas?

Background colour of title’s – I want the category box title’s and the widget title’s to have black background’s with yellow writing but I can’t find how to change this in the stylesheet. Help please! (Also want to change the font of these but am currently unable to)

Any help would be greatly appreciated!

Many thanks 🙂