Of course,
here it is.
 

Is it possible that this is because in the [variation].css like "seagreen.css" the slider-selection is defined as
.fruity .slider-selection{background-image:linear-gradient(to bottom, #84b39a 0%, #39993f 100%)}
but in the rangeslider.js there is only the "background-color"
$('.slider-selection', thisQuestion).css('background-color', opts.sliderColorM );

You see - with my very limited knowledge - I removed the "background-image" part from the "seagreen.css".
And now it looks as expected.

Normal:
 
With removed "background-image"
 

Might this cause this effect?

Best regards
Joffm

Ah, that explains! But then I tried adding

.mytest .slider-selection { background-image: none !important; }

to custom.css and it does not seem to help.

Is it possible that this is because in the [variation].css like "seagreen.css" the slider-selection is defined as...

Sure, but it's odd that I don't get that rule applied.

Anyway, I have updated rangeSlider.css to remove all background images from the slider track - github.com/tpartner/LimeSurvey-Range-Sli...geSlider.css#L37-L42

Supergreat! Now everything works, researcher is happy, the sun shines and the sky is blue! (Well, two last not really, but still, good.)