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1. I’m very unclear about how changing css will increase your website security. Css is passed to your browser, so anyone accessing you site has a view to all the css. I’m always willing to learn, but I know of no hack that involves knowing css id’s, or indeed how changing them would prevent a hack if indeed that were possible – as soon as I load your site to my browser I see all css.
2. Changing the css class bbpress would involve rewriting large portions of the plugin, which would make it unsupportibale, and would leave you vulnerable to real hacks that security releases in bbpress would prevent