I think that the right way to do this in Kuix is to use the CSS properties of the button object. Snippets of documentation:
border-color
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No
Define the border color. The value is an array of 4 colors values (like color property). Syntax : <top> <right> <bottom> <left>. If all the values are the same, you can specify only one value.
bg-image
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No
Define the background images. The value is a liste of image files path separated by a '|'.
bg-color
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No
Define the background color. The value is a color like color property.