You should look at the articles over on vb.com and the blogs. Don has written a few blogs about the style system -
vB4: Skinning Your Forum in 10 Easy Steps and
StyleVar Visual Dictionary for Forum, Blog, and CMS (PDF Documents)
As for a guide to creating mods, there are a couple of articles about writing products/plugins -
https://vborg.vbsupport.ru/showthread.php?t=82625 and
https://vborg.vbsupport.ru/showthread.php?t=82923 - but I've always found the best way to learn is to download a simple one, see how it works and try to write your own similar one and grow from there.