gtk3: Fix HdyWindowHandle separator color

Use $header_border for libhandy windowhandle separators, fixing the
color most noticeably for the Arc-Darker variant. This is similar to
how separator.sidebar is styled for .titlebar in _common.scss.

Author: jnsh

common/gtk-3.0/3.24/sass/_libhandy.scss

@@ -227,3 +227,12 @@ window.csd.unified:not(.solid-csd):not(.fullscreen):not(.tiled):not(.tiled-top):
   }
 }
 
+// HdyWindowHandle separator
+
+.windowhandle separator.sidebar {
+  &:dir(ltr), &.left, &.left:dir(rtl),
+  &:dir(rtl), &.right {
+    background-color: $header_border;
+    margin: 0;
+  }
+}