Dark theme improvements

[Gitoffthelawn]

When using a dark theme in Windows 7, some of the UI controls are not colored correctly.

If this is something you are interested in fixing, I can provide screenshots as well a theme to use for testing.

[deadlydog]

Sure, some screenshots would be nice. I'm pretty plain and use the default theme so I haven't noticed :P PRs are also welcome :)

[Gitoffthelawn]

Sounds good. I'll add getting screenshots for you to my "quick and easy" list. If you don't hear back from me within 48 hours, ping me, and I'll move it to the top of that list.

[Gitoffthelawn]

I promised you screenshots, and I'm back to deliver!

Here's how the main UI currently looks with a dark theme:

And here's how the taskbar icon looks:

The main UI works fine except for the whitish elements and the black on black buttons. Oh, and that huge 'Get Path Lengths...' button would benefit from a height reduction!

The taskbar icon will likely be fine if a brighter color is used.