Themes page is located at yoursite.mirocommunity.org/admin/themes.
To access it:
1. Select "Settings" item from the main menu on the left side of the administration interface.
2. Click on "Themes" tab.
After Miro Community Version 1.9 This area is only active for max accounts. If you have an account level that includes custom themes, you'll still be able to access and use them.