Add missing themes (from minilauta.org) and implement theme selector

[Anonyymi]

Simple HTML dropdown, clicking a theme selects it immediately. Top right corner of all pages.

• Implement the following styles
  • https://github.com/Anonyymi/minilauta-tinyib/blob/master/css/burichan.css
  • https://github.com/Anonyymi/minilauta-tinyib/blob/master/css/tomorrow.css
  • https://github.com/Anonyymi/minilauta-tinyib/blob/master/css/minilauta.css

Use the current futaba theme as a starting point. Do minimal changes, meaning only fonts, font-sizes and colors.

[Anonyymi]

Also implement yotsuba and yotsuba_blue.

[Anonyymi]

Good enough for now.