As of version 1.7, OpenMRS ships with several themes.
Go to the Admin page, click "Manage Locales and Themes" under "Maintenance".
Enter the new default theme into the themes box. As of this writing, the available options are
The colors were picked to match the colors from the OpenMRS logo.
Note: This default theme will only be applied to users that have not selected their own theme. This should be rare, however, as openmrs does not currently offer users an easy way to change their theme. (Simply adding ?theme=purple to any url would do it though). If a user does choose a theme, it is stored in a cookie on their machine and so will persist across their user sessions.
TBD. It might be possible with a module that provides files into the right folders in the webapp (WEB-INF/classes/themes).
Before 1.7 the only option as the light blue theme. An implementation can override the css and image files in the webapp with runtime properties. See "customization options" section on this page: ?Overriding OpenMRS Default Runtime Properties
Note: There is a module available for OpenMRS 1.5+ setting your own personal logo: ?Custom Branding Module