[gtk/gtk-3-24: 1/2] Adwaita: Make treeview header button borders more visible

[Jakub Steiner <jimmac src gnome org>]

commit dab79f5d7735df809d83b452a8b17d6fb39ed686
Author: crvi <crvisqr gmail com>
Date:   Mon Aug 24 01:19:47 2020 +0530

    Adwaita: Make treeview header button borders more visible
    
    The existing treeview header button border in GTK3 is close to
    invisible. The preferred way to find the border for a treeview column
    is to hover the mouse near the beginning of the header button text
    expecting the default cursor to switch to 'col-resize' cursor. This
    works only when all column headers in the view are left aligned. After
    adding a few numeric columns to the view, whose headers are right
    aligned, it becomes really hard to find the location of the border
    between a left aligned and right aligned column header.
    
    This change addresses the issue by using high contrast colors for
    borders of treeview header button - in line with the colors used in
    Adwaita theme in GTK4.
    
    See merge request GNOME/gtk!2467

 gtk/theme/Adwaita/_common.scss | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/gtk/theme/Adwaita/_common.scss b/gtk/theme/Adwaita/_common.scss
index 8e6557936e..3355266a09 100644
--- a/gtk/theme/Adwaita/_common.scss
+++ b/gtk/theme/Adwaita/_common.scss
@@ -1947,6 +1947,9 @@ filechooser .path-bar.linked > button {
 /**************
  * Tree Views *
  **************/
+
+$_treeview_borders_color: if($variant=='light',mix($borders_color, $base_color,80%),mix($fg_color, 
$base_color, 20%));
+
 treeview.view {
   @at-root * {
     -GtkTreeView-horizontal-separator: 4;
@@ -2130,7 +2133,7 @@ treeview.view {
   padding: 0 6px;
   background-image: none;
   border-style: none solid solid none;
-  border-color: $bg_color;
+  border-color: $_treeview_borders_color;
   border-radius: 0;
   text-shadow: none;