What do I do if clicking to go back to the default theme makes it crash and give me the "Unfortunately, themes has stopped." Message?
I haven't seen this issue personally, but try this:
Go to Settings -> Applications -> Application Manager -> More -> Show System Apps.
Find "Themes" on the list, select it, select Storage, and Clear Cache.*
Go back, and select "Theme Store" -> Storage -> Clear Data. Press OK on the pop-up.*
Restart your phone.
Try to apply Default again to see if this fixes it.
If it still doesn't work, try this:
Go to Settings - Applications - Application Manager, scroll down and select Theme Store -> More -> Uninstall Updates. This will install the factory-default version of the Theme Store. Once this is done, access themes through Settings - Themes, which will bring you to your list of themes on your device, and try to apply Default from there.*
If none of this works, you'll have to reach out to Samsung for help. Direct Samsung theme support can be found by going to Theme Store - More - Settings - Help - Contact Us.
*Doing these steps may remove the Theme Store shortcut on your home screen and/or App list. You can re-add it by going to Settings - Themes - More Themes - More - Settings - "Add the Theme Store icon to your app list and Home screen".