Improve accessibility according to automated reports

According to a11y accessibility checker reports, accessibility on several points need to be improved.

A bit hard to identify what a root causes. This list will presumably grow.

• Helmholtz green text has too low contrast. E.g. first headline here according to Error 8 here
• Multiple double id attributes, possibly because of re-using of same elements in header and footer (?), using <svg class="transition_slope"> blocks 2 to 3 times.
• ...?

Remaining errors should be explainable as either issues to be solved or checker artifacts by Sept 23.

Further general accessibility things are addressed in separate issues.