I haven't put this under "Suggestions" since I also present some intermediary solutions here.
It's easier to the eye; old people like myself fondly remember those very early amber-on-black screens... (which were not easy on the eye but for other reasons)
Some basic dark mode should be very easy to implement: Just allow free choice of default text color, and of default (!) background color for every pane: DONE! (i.e. leave the toolbars, dialogues, etc. as they are)
If you do LOTS of input from the SAME source (e.g. I do, hours and hours, the transcription of my own audio notes I take everywhere and "as-they-come", i.e. not in a real "dictation" format, so I have to dictate (I use Dragon Individual 15 Pro which btw works fine with UR among others) some new, "write-down" text from those notes I listen to), you might be interested in some "text input box", i.e. some third-party application into which you put your (keyboard or dictation) input, with e.g. a blank line between "paragraphs", but two blank lines between bits which would not belong together.
(Better mobile recorders, in the 3-digit range, allow for switching between recording folders indeed, but that's just some very large "subject areas", and none of them allows for immediate switching, so my note recordings come "mixed-up"; too much fiddling around with folder selection will make lose some ideas more often than not...)
So you might be ready to do the "fine distribution", into the pertinent UR folder parts, after, but you need some dark mode input box then; btw, I got the idea AFTER having realized that the above-described pc handling of my audio notes went smoother when I used just ONE, or just 3, 4 UR item(s) as my input box, instead of doing the look-up for the "correct" UR item for each such audio separately.
Some people mod (cf. "mods", "modding") some applications in some way, can't comment on this; f.lux - "software to make your life better" and free, might be such a more "general public" thing for that task, but I haven't tried.
In this inbox situation, and within UR, I need rtf formatting (my not needing that upon export being another thing), so plain text editors (e.g. Q10, with dark mode indeed), or markdown editors (e.g. Typora or WriteMonkey), are not acceptable for me in this situation (and I didn't look further into "Write!" after reading their "Install and use! You'll need to log in before you use the app, and [= seemlingly EVERY] two weeks after using it with no internet connection."; then, there are cloud apps like TinyMCE, well...), and the term "formatting", for text editors, most of the time just refers to their automatic text coloring according to elements in programming languages.
"Dark mode" isn't "full dark mode", since many applications just offer some darkness AROUND the content = text input field, whilst it's precisely there where you would want amber on black or something similar, so...
I'm not sure if the dark mode of MS Word 365 (subscription) is "full" or not, and I tried the alleged UR "competitor" (a database-driven info management application without transclusion is a joke if you ask me...) RightNote which has got a free edition, and I thought, as an inbox, just 3, 4 items in all over there, might make sense after all? Well, any "help" I could look up, referred to "dark mode" in their spreadsheets (!), and whilst I was able to set the default text to mellow yellow, and the default background to dark brown within the settings (!), I then got the yellow text on white background only, but since I have a tendency to overlook even somewhat obvious things - hello, Kyle, and sorry again for some nuisance for you, caused by that! -, that might be just my fault.
Atlantis Word Processor (35$ I think) definitely has got a real / "full" dark mode, as well as Scrivener, but that's paid, too, and 55$ or more (I just get their Euro prices, probably incl. heavy taxes) for just "half" (?!) of the functionality you'd get over at their Mac version, for the same money, would very certainly make a quite overpriced "inbox"!
Thus, I have found TWO applications which both are free, both have full dark mode, both have rtf formatting... and both are very weird, but each in its very personal way, so you really have a choice:
FocusWriter: Fullscreen-only (baaaaad!!!), and you get to the menu, etc, by moving the mouse to the upper, etc, screen border. From the menu, you then can call the themes, there you would need to duplicate a theme, and finally, within the duplicate, you'll be able to switch to some real rtf font.
SmartEditWriter (ex Atomic Scribbler), I give the link since it's difficult to find otherwise: the default GUI is a nightmare, and it's no wonder that almost nobody didn't want to buy it, but it's kind of the developer to then have made it available for free - for touting his editing tool of course, and unfortunately, I cannot endorse that since its functionality is much too poor for the price, to my liking, out there being some much more powerful editing tools if you need some -, instead of dumping it altogether. But whatever: After fiddling around for possibly long minutes with the GUI, you'll get a neat, functional, dark mode inbox for your UR staff, e.g. on the right half of your screen, toggling that with your web browser e.g. and UR taking the left half, or the other way round or however you like.
I chose AtomicScribbler, or whatever it's called now in the marketing material, to be download here:
https://www.smart-edit.com/Home/Download/ , but as said, a simple, panes-only dark mode for UR should be easy indeed. ;-) (In other words, it's not necessary to overhaul it all, for allowing free switching forth and back, but it would be up to the user to decide for dark mode, and then they would re-format their flag and other colors, etc, accordingly.)
(And, of course, Fade In has got a real dark mode, but that doesn't apply here: rtf is mandatory. Other ideas? i.e. without the crazy overhead of the Scribbler thing? But to be clear about FocusWriter: If you can live with its full screen appearance - you can switch forth and back with other applications, just not combine them with it on-screen, FocusWriter may become the UR inbox of your choice indeed.)
I did NOT try out these as an external editor for UR though, and unfortunately, and as I have explained in another thread here, Atlantis does not seem to fit as such, but at the end of the day, some external, dark mode application may be of interest for ANY part of your "workflow" in which you work on just ONE, or very few, UR item(s) for hours in a row, instead of frequently switching between multiple UR items.