Ill help you out shortly.
Bascily, its not so hard modifying vB3 style.
you need to know HTML & GFX (of course).
now when you edit it, just dont change vars that already exists.
for example:
you got main forum colors, when you change them once, they will change in all the forums (Style => All Options).
now some main templates:
Forumhome Main Page of the forum - index.php => you can find there the main build of the site.
Header i think you know what it is by its name.
Footer same as header.
threadbit threads list inside each forum.
showthread for each thread there's showthread, i mean the main build of the page, each msg have its own template which is
Postbit = in that template you can see Counters (posts) \ join date \ title \ username etc'
navbar well, i suppose i can call it "menu" as well, so basiclly this is the place that have "UserCP \ FAQ" etc'.
forumhome_forumbit_level1_nopost modifying each section header (build).
i could go on.. and on for hours, explain about all of them.
my best suggestion will be, install test forum, try it your self.
and you'll be more expert then you think in it
Hope i helped you out,
Greetz,
Fury