Welcome to the Seven Forums
You are going to need a custom theme to do what you want to do, my personal favorite is charleston2378's deviantART gallery
it's a very nice dark theme.
You will need to do 1 of these 2 ways of Enabling custom themes for it to work.
Enabling Custom Themes - Windows 7
Theme - How to Apply Custom Themes
Here are two screenshots as an example, I have changed many system files to achieve that exact look but you do not need to to get the basic look.
If you need any help installing a custom theme let me know.