[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/content.css

@@ -472,13 +472,13 @@ blockquote.uncited {
 }
 
 /* Code block */
-dl.codebox {
+.codebox {
 	padding: 3px;
 	border: 1px solid transparent;
 	font-size: 1em;
 }
 
-dl.codebox dt {
+.codebox p {
 	text-transform: uppercase;
 	border-bottom: 1px solid transparent;
 	margin-bottom: 3px;
@@ -487,11 +487,11 @@ dl.codebox dt {
 	display: block;
 }
 
-blockquote dl.codebox {
+blockquote .codebox {
 	margin-left: 0;
 }
 
-dl.codebox pre {
+.codebox code {
 	/* Also see tweaks.css */
 	overflow: auto;
 	display: block;