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

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.
Merge pull request #2707 from stephaniehobson/1058259-warning-class-conflict

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