While signed into your HonestGamers account and viewing any page on your blog, click "Settings" on the top bar to bring up a page where you can modify the general appearance of your blog, as well as your introductory bio.
You can make a selection so that your blog displays with either a white/light or a black/dark color scheme. You also can change the color of header bars that appear around the blog, and specify a background image. That image may either appear stretched across the width of your blog, or you can tile it (this works best if you have a pattern that you want to see repeat). You can also provide the URL of a 750Wx80H image to appear as your blog's header, which may be an image you submit to the site.
If you want to upload and use a custom avatar, you'll need to instead access the "Settings" page from the top bar that appears on any page of the main site.