paper-gtk-theme/Paper/gtk-3.18/widgets/_expanders.scss

/*************
 * Expanders *
 *************/
expander {
  .arrow {
    -gtk-icon-source: -gtk-icontheme('pan-end-symbolic');

    &:dir(rtl) { -gtk-icon-source: -gtk-icontheme('pan-end-symbolic-rtl'); }

    &:hover { color: lighten($fg_color,30%); } //only lightens the arrow

    &:checked { -gtk-icon-source: -gtk-icontheme('pan-down-symbolic'); }
  }
}