In order to set theme globally for application. There are two ways to do that. one is longer and second one shorter way to do that.
Create App_Themes directory for each application. This is special directory. You can copy Theme over here for each application.
Put New theme under Themes directory of aspnet_clientfile directory.
If themes directory not found then create one. For example if theme name is Green then it should be like
This will work for all website that already created. In IIS6 website can be created afterwards. so for that this clientfile does not map to new site, following location must contain themes directory and required theme folder.
After that run following command from studio command prompt
> aspnet_regiis –c
Above setting works for only IIS6 or previous versino of IIS 5.