modify new theme with Katzenpost design assets

[yippyskippy]

A couple years ago a nice logo and colors were created for Katzenpost. A hint of this can be seen at katzen.chat where the logo is also used inside the app.

Once #26 is decided and implemented, my approach would be to modify whatever theme is used with more Kat-friendly colors, fonts, icons, and maybe some fun images?!

[yippyskippy]

Small steps towards this in fac31aa but not a proper run at this.