Yeah, I'll add a spin button and a slider. And write detailed docs on how to make your own controls.
You can set images for images for back of buttons programmatically already with the code similar to the one you've posted earlier. I.e. if you have Button
button and Texture2D
texture. Then the code will be:
TextureRegion reg = new TextureRegion(texture, new Rectangle(0, 0, texture.Width, texture.Height));
Sprite sprite = new Sprite(reg);
button.Background = sprite;
Also Button has PressedBackground property(obviously it is rendered when button is pressed), OverBackground(when mouse is over it), DisabledBackground, etc.