After we updated the oldest supported WP and PHP versions, we don't need this theme anymore.
When I checked recent builds, I notice we are getting:
Error: The 'twentynineteen' theme could not be found.
So it already works for some time without the theme being activated.
[MAILPOET-5925]
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]