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The bad news is that at this level "simple" is probably not a word you'd use. But things are pretty easy considering the level of involvement.
Perhaps the easiest way to start is to make a copy of your current theme. Then open your the copy and tweak the images/HTML/CSS to look the way you want.
If that doesn't get you where you want to be, take a look at
Anatomy of the Default Theme. Also, try browsing the FAQ section. It's got helpful entries like
this.