@Laughncat 1... if you're new to WordPress theme design/editing you might want to start with learning some css before php.
Check out http://cssbasics.com for some tutorials and examples. You don't need to know php to modify the general look of a theme.
Also in the new version 0.9 that Ian just released, you can use the thematic-sample-child-theme to get started. Copy that into another folder and name it i.e. childtheme01 and you can test out what you learn from http://cssbasics.com without touching (or messing up) any of the Thematic files.
Suggestions: only change one thing at a time so you can see the effect. If you make multiple changes and something looks "weird" it will be more difficult to spot the problem(s), especially if you're new to css.
Also you should take a look at 2 excellent Firefox add-ons like Firebug and Web Developer to analyze css style sheets. You can examine/alter css parameters and see the immediate effect.
The best way to learn is by looking at live examples and the add-ons I mentioned will let you see how theme designs are put together.