Custom Design upgrade

I recently purchased a blog hosted by WordPress as well as a custom made design template. I plan on upgrading to Custom Design so that I can make changes to the blog appearance, but I noticed that you only get access to the CSS.

Is it possible to make changes to the existing template, get rid of existing text, create a unique background image, etc… (all the things you’d want to do to customize a blog) through just the CSS sheet?

I have another wordpress blog hosted externally, and with that I can just go into my ftp, find the php files, and make changes to them. I should point out i’m no expert with CSS but I do know html pretty well.