Unfortunately the Editor was not designed to have different themes, so there is no easy way to make changes to the appearance of the Editor, nor is there any documentation for doing so. All of the settings for the Editor’s appearance are in the source code. If you want to experiment with making changes yourself, most of the settings should be in the files located in the Source\Editor\EditorStyle folder.
I’m not sure what your second question was referring to. I believe you can make that change in the same place, though.
It isn’t about “dark” theme](Ironic Sans presents: Ow, My Eyes!), but about smaller brightness variation. Full white on pitch black will be very tiring. White on gray or light gray on dark gray would work better. If staring at white gets your eyes tired, then monitor brightness/contrast settings are way too high or you don’t have proper lighting in the room.
Since LCDs monitors took over, lighting is a must, because standard LCD lamp has power of military grade searchlight these days.