Fix Bug 1058259 - notification 'warning' class conflicts with Custom CSS

Defined border-width and link styles with more specificity here to over-ride the customCSS ones on .warning

You can see the conflict I'm trying to fix at https://developer.allizom.org/en-US/docs/User:stephaniehobson:test

Couldn't bring myself to add it to the customCSS as suggested on the bug. It's more maintainable here and will get minified etc.