You can add your own images to the GUI and use them as simple buttons, but you cannot override the existing controls to re-skin them or apply your own custom styles.
With the exception of the vb:button control which allows you to set a custom background colour, the GUI elements will inherit the style of the theme you currently have loaded in Renoise. We did this to help ensure that tools have a unified look and feel that matches Renoise itself.
Here are the available controls developers can choose from: (taken from the Example Tool GUI in the tool dev starter pack)
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And also some background options for adding control groups and things like that:
What i find pity is that you can’t skin your application. Transparent PNG is supported, but you can’t use that as background image and top it with controls…