Added border to notebook header

Author: lassekongo83

src/sass/gtk4/libadwaita-tweaks.scss

@@ -30,7 +30,6 @@
 // https://github.com/lassekongo83/adw-gtk3/issues/232
 notebook > header {
   background-color: var(--window-bg-color);
-  border-color: var(--window-bg-color);
 }
 
 // Better osd

src/sass/widgets/_notebook.scss

@@ -4,10 +4,10 @@
 notebook {
   > header {
     padding: 1px;
-    border-width: 0px;
+    border-width: 1px;
     background-clip: padding-box;
     background-color: $window_bg_color;
-    border-color: $window_bg_color;
+    border-color: $border_color;
 
     tabs { margin: -1px; }