Changing slider (line-)color in JS

Hi everyone,

I am simply trying to change the color of a slider to “white”. This works well when trying it offline (using Python), but I fail to get the job done online (using JS).

What I tried so far?
In the Builder I set up a slider using the style “slider” and the LineColor “white”. Since the slider does not appear as I wish (I think it has the color “LightGray”), I added a code component in which I try to change the line color.
In Python this works by adding slider.line.color = "white" at the beginning of the routine. In JS slider._setAttribute("color", new util.Color("white")); brought me the closest to the preferred outcome, but the slider still appears grayish (it looks like a combination of the colors “LightGray” and “white”).

What it should look like?

What it currently looks like?

Link to the experiment on gitlab.

PsychoPy Version
2021.1.4 (I did not update to a more recent version because with newer versions I ran into the problem that sliders did not work on smartphones)

Thank you very much in advance for your help!