Allow the composition author to provide a custom Layer (or a group of Layers) for each component of each UI widget, to completely customize the visual appearance of UI widgets.
Proposed new nodes:
Make Action Button Theme (Layers) with the following Layer inputs:
Make Toggle Button Theme (Layers) with the following Layer inputs:
Checkbox: Toggled and Hovered
Make Slider Theme (Layers) with the following Layer inputs:
Active Track: Hovered
Inactive Track: Hovered
You could then create your own Layers (using Vuo's built-in Layer nodes, or using custom images) to feed in to these nodes, and the UI widgets would display the appropriate Layer(s) at the appropriate times.