#42 Adapt viewports and colors to dark themes

[Zoitl Alois]

Currently the viewport background color, grid color, and the layer default color is hard coded. When using dark themes this makes YAPRM unuseable.

A better solution would be to use the themes background color for the viewport background and the inverted for the gird and the layer default color.

[Zoitl Alois]

implemented in [code:r500]