Slider: custom design for "is-lowest-value" should be made optional

Several of my sliders are not left-anchored at zero. They can be used to indicate user preferences on a scale between poles, such as "liberal-conservative", "masculine-feminine", "light-dark", etc.

There should really be a way to disable the custom design for "is-lowest-value". I've had to install about 50 lines of code to revert the thumb to use the normal design.