All dialogs now use default skin and foreground colour

(The attributes are still available in the schema though, and will not be removed.)

rsrc/dialogs/abort-game.xml

@@ -1,6 +1,6 @@
 <?xml version='1.0' encoding='UTF-8' standalone='no'?>
 <?xml-stylesheet href="dialog.xsl" type="text/xsl"?>
-<dialog skin='dark' defbtn='cancel'>
+<dialog defbtn='cancel'>
 	<pict type='dlog' num='23' top='9' left='9'/>
 	<text top='6' left='53' width='249' height='31'>
 		If you return to the Main Menu, your unsaved progress will be lost.