Add a dark theme

[TolikPylypchuk]

Since I'm now using FluentAvalonia which makes switching between light and dark themes trivial, this should actually be done. There are several things though that need to be implemented:

• Check that the UI looks good with the dark theme and fix anything that doesn't

• Think what to do about kinds having just one color for each item type - there should probably be a color for each theme as well, thus multiplying the number of colors by 2

• Implement switching between themes in the preferences

• Make the default theme dependent on the system theme (only for Windows)

[TolikPylypchuk]

The last task won't be implemented since it will introduce platform-specific code, and I want to avoid that to make the project simpler.