I looked through the code a little and found that the values for this are hardcoded.
I don’t understand exactly why. You could also read it using :
window.getComputedStyle(document.body).getPropertyValue(<var_name>)
via the css :root variables, and this would give Custom-Themes the chance to change this.
so there is – According to my research – currently no way to change this unless you edit the js file directly. 