OK, just a couple of quick checks –
When you say you pasted it into reboots main css file, I take it that means you are using either a child theme or the child theme framework? If not and we’re talking about reboot.php, you will need to make sure it comes after the standard rule, so best thing to do is paste it at the very end of the stylesheet (making sure it’s not inside media rules).
Is your themes custom CSS area the SP theme editor? Or do mean a WP theme ‘custom includes’ box? I’ve not tried forum CSS in the latter, but just a note – if you’re just using the stock reboot theme, the width is defined by a variable in the overlay so I couldn’t say if that would cause it to be ignored.