Custom themes
There are already a few theme options available in the Theme tab of System Settings, but you might want to have a few other color schemes available for users to choose from. To look at the themes currently available in your instance, navigate to System UI | Themes .
If you're in a Geneva or Helsinki instance, you'll notice quite a few themes that are not listed in System Settings: the additional themes are for UI15 or UI11. Here's how you can tell: Each theme has a field labeled Device that identifies the UI version that the theme applies to. A device value of Browser indicates a UI11 theme, while Doctype indicates a UI15 theme, and Concourse indicates a UI16 theme.
I know that sounds very odd, so I'll re-state it another way:
If the Device field on a given Theme record is set to Browser, then that theme only applies to UI11
If the Device field is set to Doctype, then that theme applies to UI15
If the Device field is set to Concourse, the theme applies to UI16
Let's get hands...