I am new in EP. I have created a theme but how to assign this to a particular user?
After giving credentials when you enter in the portal whatever you see is the portal desktop.
Portal desktop contains two things-
Page Framework(Layout of the screen)
Follow this path System Administration->Portal display->Desktop & Display Rules->Portal content
You can create a new portal desktop by right click and select Portal desktop.
Give the name and id for that.
then add the theme which u created and portal page framework(you can find default page framework at this path Content provided by SAP->End User Content->Standard Portal Users)
Now Open Master Rule collection. U can find it here Portal Administrators->Super Administrators. Create a rule as u wish.