Use 9-slice background in formspec theme

mods/default/init.lua

@@ -13,6 +13,7 @@ minetest.register_on_joinplayer(function(player)
 	player:set_formspec_prepend([[
 			bgcolor[#080808BB;true]
 			background[5,5;1,1;gui_formbg.png;true]
+			background[5,5;1,1;gui_formbg.png;true;10]
 			listcolors[#00000069;#5A5A5A;#141318;#30434C;#FFF] ]])
 end)