WIP: Dark theme

[KaiserBarbarossa]

Just a WIP pull request.

The dark theme would just be created if the user has enabled dark theme in his/her operating system.

[chris34]

• I would suggest to use css properties to avoid c&p of many css selectors (that will probably cause problems in the future). I have an demo HTML attached, that demonstrates what i mean (the example uses just the browser-width to change the color of the heading).
• Another problem will be wrong/low-contrast color of icons. Personally i would try to avoid multiple icon-files with just different color. https://css-tricks.com/change-color-of-svg-on-hover/ lists some ways for SVGs. I'm still unsure, whether <svg><use>…, filter:, mask: instead of backound-image or inlining SVG in CSS fits best.