Contrast issue on table headers under dark mode

[deniak]

The table headers are failing the contrast ratio requirement under the dark mode.

This is caused by the following rule in common.css:

th{
	background-color:#eee;
}

So that rule should either be removed or updated to support the dark mode.

Initially reported here.

[pkra]

Duplicate of #2394 which itself was a duplicate of #2226