As said on Discord, I propose the use of CustomTkinter as a semi-drop-in replacement. My arguments:

• It's extremely light on dependencies (darkdetect is self-explanatory and packaging is used for version detection logic; neither depends on anything else)
• The API avoids deviating from standard Tk as much as possible
• Well-maintained
• Reasonably popular (~10k stars, supported by many other libraries)
• Modernized widgets - while still supporting the original Tkinter ones
• Automatic HiDPI scaling - though the factor can be overridden if so desired
• Perhaps the most important of all: a painless theming support (fully configurable via .json files)

Option B: https://ttkbootstrap.readthedocs.io/en/latest/
Less popular but still nothing to scoff at with ~1.7k stars - and the documentation is much nicer.

The only dependency is Pillow, which is already on develop anyway.

EDIT: the latest release does not work with Pillow 10.x - which means no Python 3.12 either; although it's fixed on master (see: israel-dryer/ttkbootstrap#490), the dev has been AWOL for a while.

CTk's scaling code stumbles upon an ancient Tk bug in some locales: TomSchimansky/CustomTkinter#571

Although there's a fix on the comments, facing such an issue from the outset - after both libraries gave me hope to start tackling our UI nightmare - just makes me want to yeet anything Tkinter-based into an active volcano.