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

/**************
 * Tree Views *
 **************/
GtkTreeView.view,
treeview.view {
  -GtkTreeView-grid-line-width: 1;
  -GtkTreeView-tree-line-width: 1;
  -GtkTreeView-tree-line-pattern: '';
  -GtkTreeView-expander-size: 16;

  border-left-color: mix($fg_color, $base_color, 50%); // this is actually the tree lines color,
  border-top-color: $bg_color;                         // while this is the grid lines color, better then nothing

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

  &:selected {
    &:focus, & {
      border-radius: 0;

      @extend %selected_items;
    }

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

  &:insensitive {
    color: $insensitive_fg_color;

    &:selected {
      color: mix($selected_fg_color, $selected_bg_color, 40%);
      &:backdrop { color: mix($backdrop_selected_fg_color, $selected_bg_color, 30%); }
    }

    &:backdrop { color: $backdrop_insensitive_color; }
  }

  &.separator {
    color: $bg_color;

    &:backdrop { color: transparentize($bg_color, 0.9); }
  }

  &:backdrop {
    border-left-color: mix($backdrop_fg_color, $backdrop_bg_color, 50%);
    border-top: $backdrop_bg_color;
  }

  &.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, 70%);

    &:hover { color: $fg_color; }

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

      &:hover { color: $selected_fg_color; }

      &:backdrop { color: mix($backdrop_selected_fg_color, $selected_bg_color, 70%); }
    }

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

    &:backdrop { color: mix($backdrop_fg_color, $backdrop_base_color, 70%); }
  }

  &.progressbar { // progress bar in treeviews
    @if $variant == light { color: $base_color; }

    border: 1px solid $selected_borders_color;
    border-radius: 4px;
    background-color: $selected_bg_color;
    background-image: linear-gradient(to bottom,
                                      $selected_bg_color,
                                      darken($selected_bg_color,10%));
    box-shadow: inset 0 1px if($variant=='light', transparentize(white,0.7),
                                                  transparentize(white,0.85)),
                      0 1px if($variant=='light', transparentize(black, 0.8),
                                                  transparentize(black,0.9));

    &:selected {
      &:focus, & {
        border-radius: 0;

        @if $variant == 'light' {
          color: $selected_bg_color;
          box-shadow: none;
        }

        @else { box-shadow: inset 0 1px transparentize(white, 0.95); }

        background-image: linear-gradient(to bottom,
                                          $base_color,
                                          darken($base_color,10%));

        &:backdrop {
          @if $variant == 'light' {
            color: $selected_bg_color;
            border-color: $selected_borders_color; // otherwise it gets inherited by .view(?!?)
          }

          @else { border-color: $backdrop_base_color; }

          background-color: $backdrop_base_color;
        }
      }
    }

    &:backdrop {
      @if $variant == 'light' { color: $backdrop_base_color; }

      @else { border-color: $backdrop_base_color; }

      background-image: none;
      box-shadow: none;
    }
  }


  // progress bar trough in treeviews
  &.trough { 
    background-color: transparentize($fg_color,0.9);
    border-radius: 4px;

    &:selected {
      &:focus, & {
        background-color: if($variant == 'light',
                             transparentize($selected_fg_color, 0.7),
                             darken($selected_bg_color, 10%));
        border-radius: 4px;

        @if $variant == 'light' {
          border-width: 1px 0;
          border-style: solid;
          border-color: $selected_bg_color;
        }
      }
    }
  }

  // Column Header

  column-header,
  header {
    .button,
    button {
      $_column_header_color: mix($fg_color, $base_color, 50%);

      @extend %column_header_button;

      color: $_column_header_color;
      background-color: $base_color;
      font-weight: bold;
      text-shadow: none;
      box-shadow: none;

      &:hover {
        @extend %column_header_button;

        color: mix($_column_header_color, $fg_color, 50%);
        box-shadow: none;
        transition: none; //I shouldn't need this
      }

      &:active {
        @extend %column_header_button;

        color: $fg_color;
        transition: none; //I shouldn't need this
      }
    }

    button:last-child { &:backdrop, & { border-right-style: none; }}
  }

  // for treeview-like derive widgets
  .button.dnd,
  button.dnd,
  column-header.button.dnd,
  header.button.dnd { 
    &:active, &:selected, &:hover, & {
      padding: 4px 6px;
      transition: none;
      background-image: none;
      background-color: $selected_bg_color;
      color: $base_color;
      border-radius: 0;
      border-style: none;
      box-shadow: inset 0 0 0 1px $base_color;
      text-shadow: none;
    }
  }
}

%column_header_button {
  padding: 4px 6px;
  border-radius: 0;
  background-image: none;
  text-shadow: none;
  border-style: none solid solid none;
  border-color: $bg_color;

  &:insensitive {
    border-color: $bg_color;
    background-image: none;
  }

  &:backdrop {
    border-color: $backdrop_bg_color;
    border-style: none solid solid none;
    color: mix($backdrop_fg_color, $backdrop_bg_color, 50%);
    background-image: none;
    background-color: $backdrop_base_color;

    &:insensitive {
      border-color: $backdrop_bg_color;
      background-image: none;
    }
  }
}