The theme selection screen is accessed as follows;
1. Enter the castle editor (the "defend" button at the top of the screen)
2. Open the shop window (the yellow button with the guy (Cornelius)'s face and a shopping cart)
3. Go to defence (the tab in the shop window with the sheild)
4. Open the drop down menu (below the tabe in the shop window)
5. Select "castle themes" (the option at the very bottom)

Once you are in the theme selection screen, the themes are displayed. Themes that you do not currently own must be purchaced with blings (green gems) (usually 299). on the themes that you do own, the option to buy it is replaced with "apply". Select this, and the castle's apperance will change to that of the selected theme. The castle layout will not be changed, so will not have to validate your castle after changing the theme.