If there is a space, you put a space!
Basically, if you are setting the class for an element at the root of the component, then there is no space between THIS and the class name, but if you are setting something that is not at the root then you do need the space.
The next obvious question is what do you do if the class needs to be defined for an element at the root and not root? Great question, I am glad you asked. Here is the answer: