Revisit tweaks for mermaid after MR 2452
Some CSS tweaks introduced in !2452 (merged) for mermaid break some diagrams drawn in Mermaid in gitlab-docs.
The goal aimed in #1139 (closed)
Currently our mermaid diagrams have a bit of a springtime theme -- purple and yellow.
Would be nice if they were a bit more monochrome, and if the fonts matched the fonts in the rest of the docs.
Some was fixed in !2920 (merged). gitlab!88199 (closed) was blocked to get merged by this problem.
Edited by Takuya Noguchi