https://en.wikipedia.org/wiki/Windows_XP_visual_styles
Personally I liked most of these visual themes, but that may be biased by how snappy and responsive the UI felt compared to later entries, especially Vista, which introduces the translucent windows thing that Microsoft has kept moving forward. I know my GPU can do those effects in its sleep now, but when it first launched it was painfully slow on normal-people computers lacking the requisite graphics acceleration. That combined with Vista's initially poor Superfetch implementation (constant hard drive activity and weird pauses) and the whole modern themes thing just left a bad taste.
Ignoring Windows specific things, I'm personally fond of a UI that is easy to scan, tells me what I need to know, and gets out of the way. Electron apps that do this well don't bother me, I don't need the whole OS to agree, but I do wish that application designers choosing to ignore the system themes would at least implement a dark theme.
Alpha Blending was present since Windows 2000. You had to enable it but it was there.
In general, I find that any simple theme with clearly-visible 3D styling is good enough, and Adwaita (though maybe a bit too flashy for my preference) gives us that - with the extra bonus of being touchscreen-ready by default, which is also quite a big deal these days.
Yes, that style.
So, like, the Windows 95 / Windows 98 / Windows NT / etc type of look, or?
It’s been so many years since I used Windows XP, so I no longer quite remember what it was like exactly. I do have fond memories of using it though :)