IMO, skinning/theming will not be a big problem, based no two reasons:
1.Owner-drawn framework often has its own style/theming and provide many themes(e.g., UWP, WPF, Flutter, QT).
Yeah, but that again requires extra work to keep that up to date. And usually the similarity is only visual, and very superficial. "Feel", as PascalDragon rightly says is not emulated (I do quite a lot of application manipulation over keyword like ctrl-tab to jump over controls etc, and that usually fails miserably)
So yes, you can implement your own theming as a workaround, but that is a very large extra burden.
2.And the users doesn't always prefer native look either. Own-theme sometimes can be a selling point.
Sure, like for POS or machine software. Though that is not 100% a selling point for owner drawn, since a few different styled buttons and colors confuse most users enough to not recognize VCL apps as standard either. A few opengl widgets with some animations also throw them off.
So while true, that is a bit the "niche" application I was talking about, and it is in addition to native apps, and not instead of.
Native look doesn't always preferable for users. Just look at the 'native' ui controls of Windows, i.e., those win32 controls in WinForm/VCL/LCL. Yes, they are native, but native doesn't mean attractive, especially compare with the new Metro UI?
I don't really like the Metro UI. For desktop, the apps look like upscaled phone apps and are very bad in making the most of screen space. Keyboard navigation is next to impossible.
Also, just look at you windows desktop, how many serious softwares adopt 'native' UI looks? Almost all of them implement their own skin/theme system, either using GDI or DirectX/OpenGL.
Because most apps on your desktops are made by companies with billions in turnover that can set trends. Unless you have that kind of power behind you, it is pointless to try to imitate their decisions, because you can't emulate the main reason why they can get away with just about anything.
Only compare
Flutter uses Skia, QT/QML uses OpenGL, and they are very popular and we seldom heard the users complain about the non-native look.
Can't remember an app using flutter/skia. Most of those apps are not payed for, and usually business users are the critical ones, wich often already balk at installing extra libraries or frameworks.
You must regard this as a one developer company trying to sell a product based on the framework as the main product, and think of e.g. an office environment and intensive use. Not as e.g. Spotify or Netflix peddling a service and giving you an app for it. The service is more the attraction, and the app is only there to instrument it. YOu often only bother to look at it, after you already forked over the cash.
QT is as old as the road to Rome, and more a portable winapi than something radical different. QT is also about the only ownerdrawn framework that is somewhat done. The others never reach their goal (like own theming and decent look feel, and not being the lowest common denomitor subset)
QT (and then specially that there is only one such widgetset, and that it is commercial) is more a reason against ownerdrawn than for it. Apparently there is only room for one such kit, and it has been filled.
GTK tried to emulate it, and is still a sad, sad heap of dung.
In fact, the only problem with owner-drawn framework is it needs a lot of work.
It is not the only one (theming , often subpar look/feel and lowest common denominator in just about everything), but it is the main one.
And that lot of work is both the initial effort, and the keeping it up to date effort.