Is it better to write one own's GUI in this case? I believe most toolkits have a way to add new controls, so it should be less work to add a knob to GtK, Qt or any other toolkit.
Then you hit the second requirement: complete bitmap theming, many times of a skeuomorphic kind. This is extremely difficult to do in something like Gtk/Qt.
https://doc.qt.io/qt-5/qtwidgets-widgets-styles-example.html
2. See the diamond-shaped radio buttons in the second example. :)
3. 3D embossed arrows on scroll bars.