Aesthetic Suggestion: for the top bar of the Windows version to match the theme of the rest of the app. Like, for instance, Spotify and many other apps. (I realize Spotify uses the Chromium Framework and Scrivener uses another, so maybe this is techno-difficult. But still, would be totz adorbz).
Hiding it does not seem to be an option, since it displays the file name, which is useful.
Here is a thread on how you can change it with themes, but from the discussion there, it seems like Windows 11 may have broken it. So that aside, yes, you can change the colour in Windows settings. I do, because the default shock white is so obnoxious.
To the point though: this isn’t technically a part of Scrivener, but of Windows. Where you see otherwise, the developers have specifically removed the standard title bar and then reinvented their own.
That’s controlled by Windows Settings and part of the overall “acrylic” fluent design system, that can either be a solid colour of choice or a transparency effect on Windows 11, which applies a translucent, frosted glass look to this area as well as to various other user interface elements like the Start menu, Taskbar, Window borders, Context Menus, etc.
Scrivener’s (or QT - the development framework’s) control of colour begins below the Title Bar.
I have Wooden Walkways as my Windows Theme, which rendered a puke-like green for the aforementioned in Dark Mode, so I settled for a solid storm black colour to override the feature.
My personal preference is to do as little fiddling with my computer as possible. Sit down, type, sip coffee, type, get on with life, that sort of thing. But then I’m the one that chose Windows, so, we don’t get all we want in life :-).