Where can I learn?

[Deaths]

Congrats on your step from PHPBB to vB!

Modifying the header is a whole lot easyer than in PHPBB, I'll tell you that.

If you want to modify a style, you do so by editing templates. (Admincp -> Style Manager -> Styles & Templates).
Now, once you get to a template (header, for example), you will note that it is mostly powered by variables (the words with $ infornt of it), and css atributes (example: <td class="navbar", where the class indicates it's CSS).

If you would want to customize your skin, you can do so entirely in your Style Manager.
You can edit the CSS to change the color layout, the Stylevars to change the image directories and the header image, and so on...

I believe there are quite a few guides at vbulletin.com, so I think your best shot is to search there.

Hope this helps

EDIT: Damn you Marco! Sneaky!