feat: add theming support

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I'm submitting a ... (check one with "x")

[ ] bug report => search github for a similar issue or PR before submitting
[x] feature request
[ ] support request => You can submit support request here, but we preffer to chat about it realtime at our [ngParty slack](https://ngparty.herokuapp.com/)

Current behavior

There is no theming possible

Expected behavior

Theming implementation via sass variables during compile time or even better via css variables and ShadyCSS

Reference implementation: https://github.com/material-components/material-components-web/blob/master/docs/theming.md

NOTE: don't get confused that css variables works when you are testing on mac:

If you are using any modern browser besides Edge (which is currently working on it), you'll see that the button background as well as the focused underline and label on text fields are now a nice, dark shade of blue.