I uninstalled both Form Builder and HTML Editor. Then I reinstalled the Form Builder beta.
Now, when I try to apply an Editor, I get this error:
Error applying HTML Editor:
TypeError: paramObj has no properties.
So, I can't get it to do anything at the moment (as far as custom toolbars go... I am able to use a default style). I will continue to play around with this to see if I can resolve it.