Users can select a theme of their choice from User Preferences page of the portal and apply it.
To get the User Preferences page, click Personalize > My Preferences.
Figure 2: Select a Portal Theme
On this page, you can select a theme from Default Portal Theme drop down box.
Note: If you want the new theme to be the default one, rename existing Default with some other name, and rename theme name folders under Image folder and stylesheets folder as Default. New theme will be treated as Default one.