appearance - themes - editor

bottom right of the sceen will be Stylesheet (style.css)

You need to click that and it'll give you the options to edit, you'll need to know CSS to do a good job of what you are looking for. Or a lot of good guessing?

It's best to ftp into your site and download the css files to your computer and edit there then reupload it.

wp-content - themes - "Theme Name Here"

the style.css will be there.