What can be used to create custom GUIs?

Question

C++ / C#

I'm just looking at c#, but I already have a question that has been haunting me for some time. Tell me, is it possible to c# write non-standard gui?
So that there are no standard frames and other things, but so that everything is as I want? If possible, then with the help of what, give links to the best libraries.
And more on the topic - if you can create beautiful guis, can they be used outside of windows? And if it's different, is it possible to compile a program written for c#systems other than windows?

Answer 1

Tell me, is it possible to write non-standard guis in c#?
So that there are no standard frames and other things, but so that everything is as I want?

Yes, you can draw your own controls. Though from scratch, and on top of existing ones. This is how WPF works, by the way.
Yes, with GTK# . For example: one , two .

Answer 2

Tell me, is it possible to write non-standard guis in c#?

It’s impossible, Rostekhnadzor will come and fine you for non-compliance with GOST, demand payment of compensation to users for the extra load of RAM, CPU, GPU!
WPF, everything can be done "custom" on it, on WinForms it is also possible, but you will not adequately make a frame with shadows or transparency on WinForms, there is no hardware graphics acceleration in WinForms, but the application will start faster.
Another direction is SkinFeature, SkinCrafter.
As well as ready-made libraries, from DevExpress and Telerik, in them this is one of the functions, along with other "bells and whistles".
It is possible, but not everything will work on other systems, WPF will not be, it needs DirectX, it is not on Linux and OS X ...