This module helps you to assign multi color stylesheets to a theme and can manage/select a color from admin.
Didn’t understood, no problem simply go admin panel and give a test drive.
http://www.santoshsetty.com/tf/opencart-templates/csslayout-3col/admin/
Username: demo | Passowrd: demo
After login to admin panel, go to Extensions -> Modules and then click Edit beside to Stylesheet Switcher
Bonus
With this module you will get 13 in-built css files, which are ready to use for default theme.
Note: To use this module you need to have basic php knoweldge
OpenCart Compatibility
1.5.4, 1.5.3.1, 1.5.3, 1.5.2.1, 1.5.1.3.1 Source