It really depends on what you want to do, if your just gonna change the colors then photoshop it what you need and then just replace the old images with the new, you will also need to spend a little time in:
admincp>>Styles & Templates>>Style Manager
to match up the font colors and such
If you want to totally tranform it then this is probably the best place to learn coding:
W3Schools Online Web Tutorials
you will find all the Web-building tutorials you need, from basic HTML and XHTML to advanced XML, SQL, Database, Multimedia and WAP.