I think I looked into this some time in the past and it turned out that to do this, you need to use autoconfig scripting to inject a stylesheet as an agent style. Then, you can use style like this:
@namespace url('http://www.w3.org/2000/svg');
#box-model-elements{
--highlighter-box-content-color: hsl(297, 71%, 53%);
}
I'm limiting that style to only apply to svg elements by namespace, because otherwise the style will apply to all websites as well. But nonetheless, this would also set this custom property to all svg elements in any svg that has has that particular id. Not a huge deal, the point is simply that you need to do something to limit the scope the you want to apply changes to.