Click 'advanced style settings', then click the box under where it says "If 'custom,' select a colour here". Once you've used the colour picker to choose a colour, click 'install style' or 'update style' near the top of the page (just below the big picture).
If you're installing as a userscript (by clicking the 'Install style as userscript' link near the bottom), you won't be able to change the colours.
Comments
If you're installing as a userscript (by clicking the 'Install style as userscript' link near the bottom), you won't be able to change the colours.
If you still need help, let me know!