dots/usr/share/themes/Dracula/gtk-3.20/apps/_mate.scss

//
// Mate OSD Window
//

MsdOsdWindow.background.osd {
    border-radius: 2px;
    border: 1px solid $borders_color;
  
    .progressbar {
      background-color: $selected_bg_color;
      border: none;
      border-color: $selected_bg_color;
      border-radius: 5px;
    }
    .trough {
      background-color: darken($osd_bg_color, 5%);
      border: none;
      border-radius: 5px;
    }
  }