Just to report my experience, Tweaks didn’t cause any problems for me, although it doesn’t adapt without an adjustment such as can be done using the new Mobile Settings, which makes the display tiny, and it does have an “Appearance”/“Themes” category which offers dark and high contrast themes. You have to turn the screen to landscape view to see it.
I checked my package list, and I do have gdm3 installed.
I’m showing GNOME Tweaks in the Software store, but I don’t remember if I installed it that way or from the terminal.
You could make your own app for setting a theme: