edit: links are up-to-date for version 1.4.0
Sharing custom themes for Blender 2.5 used to be difficult, as you had to share entire .blend files, replace your default .blend and then manually reorganize your start-up screen with the new colours. That's why I'm introducing: Theme manager
Install and enable the add-on. (click here for more help)
Go the the theme panel in the user preferences.
Click the 'Load Theme' or 'Save Theme' buttons in the header (by default at the bottom of the window).
When saving a theme, you can optionally also name the theme and add the name of the creator (see video).
Themes are saved as .blt files, short for BLenderTheme. These are pretty small and can be easily shared.
You can download the add-on here: <python script>
A temporary repository with themes can be found here.