Add an OLED black theme

[rstanuwijaya]

Added an OLED black theme (for OLED display)

[BirdInFire]

As a disclaimer this review only talk about the code itself not the "idea" of oled version (which will be stated later in "conversation").

I actually find a major issue witch make me ask the refusal (if not modified) is the fact that this actual code will do nothing (as not loaded anywhere), and if a change is made to load it, it will override the normal dark theme.

So my request is :

make a change to trigger it with an optional about:config settings
add it in a way it's actually loaded by the theme.
add a change in the readme to document the feature

[BirdInFire]

So now my "PERSONAL" stance is : I don't really see the benefit of an oled version as libadwaita do not propose this actual "oled" color scheme.