Let the color for our default catalog be dependent on the color scheme

Remove catalog color for OpenNGC. Apply colorscheme-specified colors consistently in the absence of a catalog color.

Finding a color that works well across all three of Night Mode / Dark background / Light background is hard. So the proposal here is to remove the default color for OpenNGC, so that our default catalog works well in all color schemes.

The remainder of the changes are needed to consistently apply the color scheme when dynamically switching color schemes in the app (as opposed to picking up the color at the start and sticking with it)