I stopped the "Themes" service in Windows 8, but didn't notice anything.
What does it do?
In Windows 7 is it described as "Provides User Experience Theme Management"
I doubt it has changed in W8, try changing the theme with the service disabled, it will probably not work. Right click on desktop and select "personalize" to change the theme.
According to this article sometimes you will have trouble changing/applying new themes if the service is active, you have to restart the service to solve it, remember W8 is BETA software and will have tons of bugs.