RiverLea Thames Dark Mode / Extensions colour fix

[vingle]

Port-forward (Forport?) of #32370 in response to @Stoob's issue: https://lab.civicrm.org/dev/core/-/issues/5793

Overview

This fixes a regression caused by #31994. I tested that PR and made a bunch of changes to regressions in #32209, but hadn't tested Extensions layout in Thames dark-mode, so am adding this. @artfulrobot feel free to change/resubmit your own, trying to be helpful here.

I'm going for the simplest fix - change bg to be dark, using a shade of green used elsewhere in Thames/DarkMode.

More info and screengrabs - please see the first PR.

[civibot]

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[ufundo]

I think its a forward port of #32381 but apart from that looks good @vingle