To install a new theme, select one of the .tmTheme links listed below, download it to your desktop and double-click on the file. This will place it in ~/Library/Application Support/TextMate/Themes, at which point the new theme will appear in the list found in Preferences → Fonts & Colors. This will also set it as the active theme. (Once the theme is installed, you may remove the .tmTheme from your desktop.)

Rails Envy TextMate Theme
http://railsenvy.com/2008/9/18/rails-envy-textmate-theme

User Submitted Themes
http://wiki.macromates.com/Themes/UserSubmittedThemes