Enable dark mode

Job Doesburg
2020-01-30T13:03:58+01:00

Closes #953

### Short description
A lot of CSS has been refactored to easily support CSS `prefered-color-scheme`s. All regular SCSS (except things in the admin, or some special exceptions, such as signature fields) should now use variables (like `$preset` or `$preset-contrast`, `$background-color`, etc.). Then in `_variable-usage.scss` files specific to the chosen color scheme, those variables are mapped to real colors in ~_variables.scss`.

Therefore in `_variables.scss` we define all settings specific to our identity, and in `_variable-usage.scss` we can use those settings to create a color scheme.

### Steps to use this feature

1. Visit the website, from a agent that has `prefered-color-scheme: dark`