gnome-shell/data/theme/gnome-shell-sass/widgets/_slider.scss

/* Slider */

$slider_size: 15px;

.slider {
  height: $slider_size;
  // slider trough
  -barlevel-height: 3px; // has to be an odd number
  -barlevel-background-color: $borders_color; //background of the trough
  -barlevel-border-width: 1px; 
  -barlevel-border-color: $borders_color; // trough border color
  // fill style
  -barlevel-active-background-color: $selected_bg_color; //active trough fill
  -barlevel-active-border-color: if($variant == 'light', darken($selected_bg_color, 4%), lighten($selected_bg_color, 2%)); //active trough border
  // overfill style (red in this case)
  -barlevel-overdrive-color: $destructive_color;
  -barlevel-overdrive-border-color: if($variant == 'light', darken($destructive_color, 4%), lighten($destructive_color, 2%)); //trough border when red;
  -barlevel-overdrive-separator-width:1px;
  // slider hander
  -slider-handle-radius: $slider_size * 0.5; // half the size of the size
  -slider-handle-border-width: 1px;
  -slider-handle-border-color: if($variant == 'light', $borders_color, $fg_color);

  color: if($variant == 'light', lighten($bg_color, 10%), $fg_color);
  &:hover { color: $hover_bg_color; }
  &:active { color: $active_bg_color; }
}