I’d just like to interject for a moment. What you’re refering to as Windows, is in fact, Adware/NT, or as I’ve recently taken to calling it, Adware plus NT.
Please put an NSFW tag on this. I was on the train and when I saw this I had to start furiously masterbating. Everyone else gave me strange looks and were saying things like “what the fuck” and “call the police”. I dropped my phone and everyone around me saw this image. Now there is a whole train of men masterbating together at this one image. This is all your fault, you could have prevented this if you had just tagged this post NSFW.
A Free Software License is even more important. There are many great projects out there which you can’t modify etc. because the project isn’t distributed with a license (which means “all rights reserved” in most jurisdictions).
Please don’t automatically generate themes for third-party apps. If an application brings its own styles and icons, it results a weird mix of multiple styles.
If a user wants to style it themselves, they should be able to — at their own risk. But shipping (inherently broken) styles with a distro/DE misrepresents the appplication and creates unnecessary issues for the upstream developers.
GNOME Extensions actually run in the gnome-shell process itself and can do most things that a builtin solution could offer.
They fail in some circumstances […] and behave inconsistently
That proves why they shouldn’t be part of GNOME Shell themselves. Offloading some (debatable) functionality to extensions helps keeping the core components reliable and maintainable.
The file selection dialog is not a part of Nautilus. It is either a provided by the toolkit (e.g. Qt, GTK3, GTK4) or by a xdg-desktop-portal implementation. The GTK4 file chooser that is also used by GNOME’s portal implementation supports thumbnails since December 2022 or GNOME 44.
I guess you are using an older (LTS) version of ubuntu that uses an outdated version of GTK.