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.

@Friendica Support i really wish there was another option for having a dark theme in #Friendica. i would like having a multi-shade black / charcoal page, with lavender / violet accents, but so far have not found a way to achieve it (note, for many years i used to use the #Stylus AddOn, with which i created numerous custom themes for many sites, but a year ago i disabled it & others as part of my desire to begin minimising all non-essential AO attack surfaces). Though in many ways using #DarkReader for Friendica is quite good, overall i tend to eschew it being on for this site, coz it hides that handy temporary shading highlight of posts in a thread for which a Notification was received.

The small number of current dark themes offer me a rather unappealing colour choice.

The custom theme option initially excited me, til i realised, afaict, it is based on the Light theme, with no Dark theme option, so that all the dialogue boxes & dropdown menus remain eye-piercingly bright white, that hurts my eyes. The least-worst i could conjure was

{"nav_bg":"#54207b","nav_icon_color":"#f3d0f1","link_color":"#5b38a6","background_color":"#5b4d71","background_image":"","contentbg_transp":"42"}

inevitably then, i keep returning to the so-called Black theme, but it is... quite boring. the entire page is simply all a single "shiny" black tone, with my chosen accent relieving the monotony. compare this to the so-called Dark themes, to see the difference; they use different shades of the base colour, which is more visually appealing, creating a "panelled" effect.