Add method that tells the app/backend that we are using a dark mode theme

[gcasa]

The purpose of this PR is to add a method to make the system aware that a theme using "dark mode" is being used. This will assist in settings for editors and, particularly, with printing.

[rmottola]

A discussed via voice, I think we need two properties:

• Theme "is dark". When switched system issues dark mode notification and identifies as darkmode.
• Theme supports both light and dark modes

The latter porperty is for future work, non need to implement it now.

isDarkMode in this context is a little bit ambiguous.