Add icons for dark theme

[onionltd]

If a user uses a dark theme icons are not visible. What icons are used is specified by the following structure:

"theme_icons": [{
    "light": "icons/geo-16-light.png",
    "dark": "icons/geo-16.png",
    "size": 16
  }, {
    "light": "icons/geo-32-light.png",
    "dark": "icons/geo-32.png",
    "size": 32
  }]

[onionltd]

I've adjusted a tone of existing icons and changed color of the neutral bird to gray blue. This somewhat fixes the issue, tested with the default dark theme in Tor Browser.

I'm going to leave it open on off chance someone comes along and complains about it.