Change to Materials theme
- change to
materialstheme as discussed and approved in !167 (closed). - contrary to !167 (closed), this MR's content is mostly unchanged w.r.t master.
- Cherry-picked formatting-only stuff from !167 to this MR
- Later content changes, e.g. from !163 (merged) will be applied on top.
Edited by Uwe Jandt (DESY, HIFIS)