This is a topic I've avoided posting for a while, mostly because I didn't want to see a "Who cares?" or "That's not important" response. I personally think theme (as it pertains to sound and graphics) is a huge part of any game. A general theme should be decided and followed.
"Fun"I think this is the style followed by V1/V2. Mostly bright and colorful objects. Everything is usually pretty well-lit. Monsters have graphics that aren't really serious. Player graphics are kind of childish and often a bit messy. Doesn't really seem like any specific theme was in mind when the graphics were created and anything/everything was used. I feel bad in saying this because I honestly hate to say anything bad about V1/V2 but.. that's how I feel.
The sounds were OK, mostly non-existent.
(yes, I know this is a very old screenshot but I don't think the 'theme' changed much)
"Dark/Medieval"Some of my favorite games were made with this in mind. Not necessarily "medieval" as in historically accurate medieval, but the whole knights and wizards feel. Usually rougher style of graphics and a bit darker. It's not often you'll see bright objects in this theme and light sources play a huge role. The music in these games is HUGE. It's probably at least 30% of the feel. Just listen to any of the music from the games listed and you'll know what I mean.
Dransik Classic
Diablo
"Fantasy"Much more of a soft feel to it. Not necessarily the opposite of the "Dark/Medieval", but more glowing effects. Lots of ambient sounds, maybe a stronger focus on lighting effects like fog.. kind of a mysterious feel to it.
I can't think of a game off the top of my head that follows this theme, but here's a picture I found by doing a google image search on 'fantasy'.
Anyways there's obviously a bunch more themes but I think one should be at least decided upon since basically everything created for graphics and audio kinda depends on it.