Default style, but dark?
 

I really have no problems with the basic vBulletin site for now, but I just don't like how bright it is. However, manually going into control panel and tweaking colors makes the whole thing look gross. What I want is the basic default style but much darker, like black and gray with white text. Is there an easy way to do this other than pure guessing when I'm in admin CP?

Only you need to do is edit your style CSS. You can also find help here: http://www.vbulletin.com/docs/html/color_maps_intro. :)
You can also use gradients.. for make it more pretty.

Do you know of any site where I can grab modifications of the basic style? Right now I'm changing values, saving, refreshing... and the end result is ugly. Like the design of vbulletin.org is pretty nice except I'd like maybe a dark gray instead of white background, and then white text on top of it... not sure if that would look nice, but the basic is too bright for my site's theme.

You could use a free style provided by vB unofficial sites and then use CSS for change its color.

Some good sites...

www.forumtemplates.com/vbulletin/
www.forumshelp.com/
/www.forum-lance.com/

Etc.