Hi, I fixed this another way. Here is my post about this:
http://forum.pfsense.org/index.php/topic,10975.0.html
(No idea if editing the all.css file for Nervecenter will fix the issue as I don't have Opera but you could always give it a go)
If you want the exact steps, here they are:
1 - Go to Diagnostic: Edit file (you should still be able to click this one)
2 - In the Save/Load from path box, type in /usr/local/www/themes/metallic/all.css and click Load
3 - Scroll down until the / Style the List / section.
4 - Change the line that reads width: 800px; to be width: 802px; and click Save
5 - Clear your FF cache and reload the pfSense page to check it worked