Fix PRE tag in styles.css
Now PRE HTML tag is rendered in Arial.
Although it is not a W3C violation as it _recommends_ use of monospaced font for this tag, as it is used on code conventions page, it makes difficult to see exact indentation rules.
This patch defines font for PRE as monospaced.
If this page is not applicable (PRE tag is also used in signature on status page), then another solution would be to change PRE tags to CODE in web/docs/conventions.xml file.
Ticket imported from: #822332. Ticket imported from: patches/393.