[MAILPOET-5638]
I want to avoid the SettingsController becoming some kind of a god class so in this step, I extracted themejson-related stuff to an extra class. In the future, we should also move methods for manipulating CSS to a helper. [MAILPOET-5741]
[MAILPOET-5816]
To avoid repetition I decided to move this method to better place. [MAILPOET-5705]
