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OK I can understand that you're confused. But the best way to change how a theme looks, is to experiment with an existing theme. In your XOOPS install, you should have three existing themes: Default, phpkaox and X2T. In each theme folder you should see css files. My suggestion would be to change things in the css file for one theme (phpkaox is a good one)so you can see what effect it has on the theme, until you get an idea what each element in the css file does. If you do a search for "commented CSS" you shoyld be able to find a copy of a CSS file with comments added to assist you with what you need to change.
By the look of the theme you want to do, it shouldn't be TOO difficult.