The following controls allow you to control the background colour: rotary, rotary_horizontal_drag, rotary_horizontal_vertical_drag, rotary_vertical_drag
The following controls do not allow you to control the background colour: linear_bar, linear_bar_vertical, linear_horizontal, linear_vertical
(The colour is transparent, and cannot be changed, even though there is a parameter for it: trackBackgroundColor= (it is ignored, btw)
Could this be implemented, please?