You can set the map background color with the Options.BackgroundColor property.
Unfortunately it's currently not supported to make the background transparent.
However this is a good suggestion and we'll consider adding this feature in a future version.
In my TMS Web Core application the map property "Options.BackgroundColor" only does half the job.
E.g. when I set it to blue , I sometimes see blue areas flickering when the map is redrawn, but as soon as the redraw finished, no blue is visible any longer, because it's overlapped with the map content.
The missing 50% are the regions above and below the map when it is zoomed out.