Step 1: Get a decent browser with DOM inspection tools. Firefox + Firebug can do this, or Chrome plus its built in inspector tool will let you see what CSS controls those things.
Step 2: Change them!
In most SMF 2 themes, those things are controlled by images. Using the above tools you will be able to see which images these are and what parts of the images in question, which should help you get started on making the changes
(I'd sort of rather give you the tools to enable you to make changes rather than simply say 'go here, do this', because this way you'll be able to experiment and try more adventurous things for yourself