Add KDE Breeze Dark theme

[buckmelanoma]

This PR adds a new theme that follows the style from KDE's Breeze Dark.

[jeffvli]

Is the titlebar supposed to be white for this theme on Windows/macOS?

I'm thinking it should be changed to match the KDE titlebar color like what you have shown in your screenshots.

What is currently set:

titleBar: { 
  color: '#31363B', <--- change this?
  background: '#FCFCFC',  <--- change this?
},

I added a commit onto your branch so you'll need to do a git pull on your local branch before making any more commits.

[buckmelanoma]

Thanks, looks like I had the color swapped for the windows titlebar. Works for me on KDE now:

[buckmelanoma]

Btw, here's a screenshot of KDE's dolphin file browser for color reference:

[jeffvli]

Perfect!