Google has released theme feature for it’s Gmail application that enable users to change appearence or look and feel for it’s mail interface. This feature can be accessed under Gmail settings on Theme tabs.

Theme options
However, for mail application hosted in Google Apps, this feature is not enabled by default. After doing a bit searching, i found a solution that worked on my mail application.
To enable theme feature:

Domain Settting

Enable Theme
Note:
Sometimes it takes time for the changes to be applied, so just wait for it…
“New Services and Pre-release Features option”
Where is that?
Manage this Domain->Domain Setting->Email->Themes ( Please tick “Let users choose their own themes.”)