Don't use libadwaita headerbar bg. Fixes #247

Author: lassekongo83

gtk/src/adw-gtk3/gtk-3.0/_apps.scss

@@ -1159,7 +1159,7 @@ geary-conversation-viewer#conversation_viewer {
     box-shadow: none;
     &:backdrop {
       background-image: none;
-      background-color: $headerbar_bg_color;
+      background-color: $new_title_bg_color;
     }
   }
 }

gtk/src/adw-gtk3/gtk-3.0/_colors.scss

@@ -110,7 +110,8 @@ $disabled_headerbar_fg: gtkmix($headerbar_fg_color, $headerbar_bg_color, 50%);
 $backdrop_headerbar_fg: gtkmix($headerbar_fg_color, $headerbar_backdrop_color, 50%);
 $disabled_card_fg: gtkmix($card_fg_color, $card_bg_color, 50%);
 $disabled_popover_fg: gtkmix($popover_fg_color, $popover_bg_color, 50%);
-$incognito_bg_color: gtkcolor(incognito_bg_color); // adw-gtk3 specific, for gnome web
+$incognito_bg_color: gtkcolor(incognito_bg_color); // adw-gtk3 specific: gtk3 gnome web
+$new_title_bg_color: gtkcolor(new_title_bg_color); // adw-gtk3 specific for a few selected apps
 
 // titlebuttons
 $titlebutton_normal: gtkalpha(currentColor, .1);

gtk/src/adw-gtk3/gtk-3.0/_defaults.scss

@@ -40,7 +40,7 @@
 @define-color view_fg_color #{if($variant == 'light', #333333, white)};
 
 // Header bar, search bar, tab bar
-@define-color headerbar_bg_color #{if($variant == 'light', #ffffff, #303030)};
+@define-color headerbar_bg_color #{if($variant == 'light', #ebebeb, #303030)};
 @define-color headerbar_fg_color #{if($variant == 'light', #2f2f2f, white)}; // adw-gtk3 light theme note: replacing transparentize(black, .2) with equal static color
 @define-color headerbar_border_color #{if($variant == 'light', transparentize(black, .2), white)};
 @define-color headerbar_backdrop_color @window_bg_color;
@@ -76,3 +76,4 @@
 
 // adw-gtk3 specific
 @define-color incognito_bg_color #{if($variant == 'light', #d7e3f0, #1c2438)};
+@define-color new_title_bg_color #{if($variant == 'light', #ffffff, #303030)}; // because it's only used in a few selected apps