Implement time based theme switcher

[iammatthias]

Two color schemes exist for this site, a light and a dark.

• Light: https://github.com/iammatthias/net/blob/master/src/styles/themeLight.js
• Dark: https://github.com/iammatthias/net/blob/master/src/styles/themeDark.js

Goal: default to light theme. Switch to dark theme programmatically based on local time of day. Must work with current theming using Emotion.sh.

[iammatthias]

Achieved with @media (prefers-color-scheme: dark), waiting on wider browser support(implemented already, but won't work in most browsers yet).