Helpbox: use current shadow color

Origin commit data
------------------
Commit: bd57627b8f
Author: Thilo Graf <dbt@novatux.de>
Date: 2016-10-07 (Fri, 07 Oct 2016)

src/gui/widget/helpbox.cpp

@@ -45,7 +45,7 @@ Helpbox::Helpbox(	const string& Title,
 						CC_SHADOW_ON,
 						COL_MENUCONTENT_PLUS_6,
 						COL_MENUCONTENT_PLUS_0,
-						COL_MENUCONTENTDARK_PLUS_0)
+						COL_SHADOW_PLUS_0)
 {
 	page = 0;
 	hbox_y = 1;