Dark color theme with flatui colors from https://flatuicolors.com/.
The theme structure has been borrowed from grandshell-theme, which
borrowed it from color-theme-sanityinc-solarized.

URLs:
grandshell-theme: https://github.com/steckerhalter/grandshell-theme
color-theme-sanityinc-solarized: https://github.com/purcell/color-theme-sanityinc-solarized