Make form range track background more contrasted (twbs#39095)

Author: julien-deramond

scss/_variables.scss

@@ -1043,7 +1043,7 @@ $form-select-transition:          $input-transition !default;
 $form-range-track-width:          100% !default;
 $form-range-track-height:         .5rem !default;
 $form-range-track-cursor:         pointer !default;
-$form-range-track-bg:             var(--#{$prefix}tertiary-bg) !default;
+$form-range-track-bg:             var(--#{$prefix}secondary-bg) !default;
 $form-range-track-border-radius:  1rem !default;
 $form-range-track-box-shadow:     var(--#{$prefix}box-shadow-inset) !default;