@rperce That was VERY helpful! Thank you!
@enbee I’ve made some changes to the colour scheme.
The blue bookmark button when it’s been clicked on and the blue colour in the calendar date picker should be clearer now. Both problems were the same in terms of colours, so both have the same fix, which was making the blue lighter.
The on-hover effect for the profile icon > menu dropdown textual links on web should be a lot better - I made the on-hover colour darker.
The on-hover and selected effects for the profile icon > menu dropdown icons are a little better. The colours do pass WCAG AAA contrast. I would have liked to increase the contrast more but can’t do that without affecting other things in less-good ways. I made the selected colour a bit darker (as well as the on-hover colour, from the previous point). The icons have the same colour as plain text (i.e., white) and it’s probably best that I don’t mess around with that much.
For the “bugabee” category colour, as you say it’s not a darkmode-specific problem. The category colours can’t be changed as part of theme changes, so I’d either have to change the bugabee category colour itself for all themes (which I don’t want to do since it might break our habitual recognition of the category) or I’d have to change the dark-mode page background so that it’s less dark, which then has knock-on effects for everything else. So I’m very much in favour of your assessment that it’s “probably too irrelevant in general to warrant addressing” - thank you! 
More feedback is welcome from anyone, even if it conflicts with what I’ve written here. Some of what I’ve done might have had unpleasant side-effects elsewhere on the site that I haven’t noticed yet, so any reports of ugly / unclear appearances may be helpful. I don’t guarantee that I can make things much better, but I can look into any remaining issues.
Here’s the new colours:
“Dark Mode Color Palette”: {
“primary”: “ffffff”,
“secondary”: “222222”,
“tertiary”: “88E7FC”,
“quaternary”: “fcaf3e”,
“header_background”: “222222”,
“header_primary”: “ffffff”,
“highlight”: “8f5902”,
“danger”: “e45735”,
“success”: “73d216”,
“love”: “fa6c8d”,
“hover”: “3E3F3B”,
“selected”: “545954”
}