I'm also new to smf (I mostly use vBulletin), but I would think that you could probably do a search for smf forum styles/themes. vBulletin has an official mod site that contains plugins as well as styles/themes.
When I first started out customizing my theme I would search for a theme that closely resemble the look and feel I wanted and then tweaked it to my liking. There was a lot of trial and error and to some extend it's still that way, but after a while you'll get better at it and it will become easier. Eventually, you'll not only be able to change colors, but create a new logo and header, even the layout.