well well well, i have synthesised a pretty comfy workaround. the basic problem ofc remains, which is why i call this a workaround not solution.

• my daily browser is :firefoxnightly: , hence can/does use the Mozilla-created AddOn #SideView (which tbh i always misremember as being #OpenInSidebar)
• i have reverted my #Friendica Settings Display theme to the default Light [ow ow ouch, my poor eyes!]
• i have re-enabled the AO #DarkReader for this site (eyes go phew!)
• once again now, per my OP, if i access my received Friendica Notifications in the main page, Dark Reader entirely hides the momentary shading / highlighting of the applicable post... ie, most unhelpful
• however, when viewing my account in the sidebar, via Side View, the Dark Reader AO does not work, so i see only the underlying Light theme. this is usually a large disadvantage IMO of using Side View, but in this specific scenario, it is actually handy...
• if, hereafter, i open my account Notifications not any more in the main page, but in the sidebar, still being the Light theme means i now DO get to see that helpful momentary shading / highlighting of the applicable post
• yes it is blinding, but less so than the main page coz its area is much smaller, plus i am only in that sidebar view for seconds, after which i revert to its default, being #TreeStyleTab.
• so far, this new workflow seems quite good

fwiw.