Change / redesign theme for grav docs

We would like to further customize our already customized theme for grav docs based on a design.

This is how it currently looks:
https://docs.luckycloud.de/en

And this is how it should look:
https://xd.adobe.com/view/449a3682-91e7-4c8d-b96e-915f3ed93cde-11c9/


This is the source code of grav
https://github.com/getgrav/grav