one-dark/gtk-3.0/sass/ui/treeview.scss

//
// Tree Views
//
treeview.view {
  @at-root * {
    -GtkTreeView-horizontal-separator: 4;
    -GtkTreeView-grid-line-width: 1;
    -GtkTreeView-grid-line-pattern: '';
    -GtkTreeView-tree-line-width: 1;
    -GtkTreeView-tree-line-pattern: '';
    -GtkTreeView-expander-size: 16;
  }

  border-left-color: transparentize($fg_color, 0.85); // this is actually the tree lines color,
  border-top-color: transparentize(
    black,
    0.9
  ); // while this is the grid lines color, better then nothing

  rubberband {
    @extend rubberband;
  } // to avoid borders being overridden by the previously set props

  acceleditor > label {
    background-color: $selected_bg_color;
  }

  &:selected {
    &,
    &:focus {
      border-radius: 0;
      border-left-color: mix($selected_fg_color, $selected_bg_color, 50%);
      border-top-color: transparentize($fg_color, 0.9); // doesn't work unfortunatelly

      @extend %selected_items;
    }
  }

  &:disabled {
    color: $insensitive_fg_color;

    &:selected {
      color: mix($selected_fg_color, $selected_bg_color, 40%);
    }
  }

  &.separator {
    min-height: 2px;
    color: transparentize(black, 0.9);
  }

  &:drop(active) {
    border-style: solid none;
    border-width: 1px;
    border-color: mix($fg_color, $selected_bg_color, 50%);

    &.after {
      border-top-style: none;
    }
    &.before {
      border-bottom-style: none;
    }
  }

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

    color: mix($fg_color, $base_color, 50%);

    &:hover {
      color: $fg_color;
    }

    &:selected {
      color: mix($selected_fg_color, $selected_bg_color, 70%);
      &:hover {
        color: $selected_fg_color;
      }
    }

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

  &.progressbar,
  &.progressbar:focus {
    // progress bar in treeviews
    color: $selected_fg_color;
    border-radius: 2px;
    background-color: $selected_bg_color;

    &:selected,
    &:selected:focus {
      color: $selected_bg_color;
      box-shadow: none;
      background-color: $selected_fg_color;
    }
  }
  &.trough {
    // progress bar trough in treeviews
    color: $fg_color;
    background-color: $trough_color_treeview;
    border-radius: 2px;
    border: 1px solid $button_border;

    &:selected,
    &:selected:focus {
      color: $selected_fg_color;
      background-color: transparentize(black, 0.8);
      border-radius: 2px;
      border-width: 0;
    }
  }

  header {
    button {
      $_column_header_color: mix($fg_color, $base_color, 80%);

      padding: 3px 6px;
      font-weight: bold;

      color: $_column_header_color;
      background-color: darken($bg_color, 1%);
      background-image: none;
      border-style: none solid solid none;
      border-radius: 0;
      border-color: $borders_color;

      &:active,
      &:hover {
        background-color: lighten($bg_color, 2%);
      }
      &:active:hover {
        color: $fg_color;
      }
      &:disabled {
        border-color: $bg_color;
        background-image: none;
      }
      &:last-child {
        border-right-style: none;
      }
    }
  }

  button.dnd,
  header.button.dnd {
    &,
    &:selected,
    &:hover,
    &:active {
      transition: none;
      color: $selected_bg_color;
      box-shadow: inset 1px 1px 0 1px $selected_bg_color, inset -1px 0 0 1px $selected_bg_color,
        inset 1px 1px $base_color, inset -1px 0 $base_color;
    }
  }
}