message: tweaked slider rail/handle color
diff: &-rail {
  position: absolute;
  width: 100%;
  - background-color: $border-color;
  + background-color: $control-background;
  height: 4px;
  border-radius: 2px;
  }
  cursor: pointer;
  border-radius: 50%;
  background-color: $font-color;
  - border: 4px solid $border-color;
  + border: 4px solid $control-background;
  &:hover {
  background-color: $hover-font-color;
  