Copy an existing .tmTheme file to your User folder.
Rename the file.
Open this file and change the 'name' of the theme towards the top of the file.
You should alter the uuid number at the bottom of the file, although I don't believe this is essential because, as I understand, this value is not used anywhere currently.
The colours are based on 'scope'. Use Ctrl-Shift-Alt-P to read the current scope in the status bar, or install the **ScopeHunter **Package.
Choose your theme from the Preferences menu.