Can I change my OS X theme?
Out of the box there is no themeing capability in Mac OS X, you can however modify some of the colours used by Mac OS X in the Appearance Preference Pane in System Preferences.

If you're serious about themeing however there are 3rd party applications you can get to help with this:


CandyBar - manages and changes icons.
ThemePark - does most everything else. If you're interested in available themes that work with ThemePark check out MacThemes


Otherwise a fairly good resource is the forum at MacThemes.