Allow changing Material UI Theme (fonts, colors, ...)

[natterstefan]

Feature Request

Developers should be able to change fonts, colors and other settings provided by Material-UI's theme. This was initially asked here #21 (comment).

Basic example

• https://codesandbox.io/s/jzn6r7r4l9 (source: https://material-ui.com/customization/themes/#palette)

Motivation

Customization of the App