Some time ago there was an idea by a user to add image processing (brightness and contrast changes, color changes) to the tiles after loading. But I rejected it because i don't know whether map providers would be happy when somebody modifies their maps and because I was worried about speed loss and about opening a box of worms with endless user wishes for more and more image variations.
One possibility how you could achieve this would be to add a new layer with a "pseudo-provider" providing just black tiles. Using the "Opacity" propery of a layer you could then reduce the brightness of the base map by increaing the Opacity of the black layer from 0 (fully transparent = original brightness) to, say, 0.5 (the semitransparent cover layer darkens all colors by 50%).
These black tiles could be created on the fly during loading since tiles, essentially, are just ordinary bitmaps. Some work, however, will be required to find where the MapViewer will refuse to load non-disk files.
@Dzandaa: The "TransparentMap" property is meant for tiles with built-in alpha channel, such as those available from the OpenRailwaymap providers: they have a fully alpha-transparent background and overlaid lines with railway tracks. When you load these maps into the MapViewer you will normally see a black background because the alpha channel is neglected by default. But when you enable the TransparentMap property the map will become transparent, and you'll see the InactiveColor of the MapView as background.
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