Dark theme

[Chirunos]

By running the program in a desktop environment with a dark theme set and looking at src/mainwindow.ui, it seems that colors are hardcoded to a light theme.

It would be nice if the program supported a dark theme as it would integrate better with more desktops and save people's eyes at night. On laptops and other mobile devices, it would also save precious battery life that could instead be used for being a node on the network for longer (lol)