Remove rem units from svg

[michjnich]

Description

Removes the "1rem" height and width from the svg attributes for the dark/light icons and puts it into css height and width attributes instead

Fixes #1935

Checklist:

• I have added the relevant tests for this change. (Note: No tests added)
• I have added an item to the Pending section of docs/changes.rst.

[michjnich]

Quick note: I couldn't run full tests here without setting up a load of databases locally (side note, might be an idea to include a compose file that sets up the various dbs for the full tox run, to make life simpler), and since this was a pure display change I figured that was probably OK :)

[tim-schilling]

Looks good! I'll pull this down, verify it's right then merge.

We don't expect people to run the tests locally against all DBs unless that's what the change is relevant for. Testing against sqlite is sufficient.

[tim-schilling]

Thank you @michjnich!

[michjnich]

And thank you for a pleasant contributor experience :)