[ticket/8228] Fix still existing problems with code in firefox

A user of a board mentioned that there are still some problems
in firefox if php-highlighting is on. So I used a snippet which
they worked on for weeks. Link to the snippet (in german):
http://www.ongray-design.de/forum/viewtopic.php?t=541

PHPBB3-8228

phpBB/styles/prosilver/theme/colours.css

@@ -470,16 +470,16 @@ blockquote blockquote blockquote {
 }
 
 /* Code block */
-dl.codebox {
+.codebox {
 	background-color: #FFFFFF;
 	border-color: #C9D2D8;
 }
 
-dl.codebox dt {
+.codebox p {
 	border-bottom-color:  #CCCCCC;
 }
 
-dl.codebox pre {
+.codebox code {
 	color: #2E8B57;
 }