Improve legibility of grid and element labels, particularly when rendering on top of light backgrounds with dark text

[mjrusso]

It can be hard to read grid and element labels in a variety of circumstances, but especially when Scoot is rendering on top of light backgrounds with dark text.

(This issue primarily affects grid-based nav, but the same technique should also be used to improve the drawing of bounding rects for elements when using element-based nav. For an entirely different legibility issue with element-based nav, see #2.)

Is there a (performant) way to modify the colours of the grid based on the content immediately underneath it?

[mjrusso]

Added a stopgap to improve legibility when using element-based navigation, until this particular ticket is implemented.

[mjrusso]

#21 added some contrast controls (which now persist across app launches), which should help (a tiny bit) with legibility.