The themes getting disabled seems to be quite the bug!
I had it just now - I found it was because I had a buggy version of Norton.
Start-> Run->(type) services.msc
Click on themes
Manually stop and start
And my themes were restored.
Although the cause of your problem seems to be diff - this is worth trying.
I tried double clicking Themes in Services and setting
Recovery tab ->First Failure (dropdown) : Restart the service
General tab -> Startup type (dropdown) : Automatic
But that didnt work.
Manual restart of the service did.
All I'm saying is - its worth trying