[ticket/12292] Improve style details page

http://tracker.phpbb.com/browse/PHPBB3-12292
Gave style details page a proper heading and back button.
Also removed a few obsolete code leftovers in the style
and CSS.

PHPBB3-12292

phpBB/includes/acp/acp_styles.php

@@ -530,6 +530,9 @@ class acp_styles
 			return;
 		}
 
+		// Show page title
+		$this->welcome_message('ACP_STYLES', null);
+
 		// Show parent styles
 		foreach ($list as $row)
 		{