Make the generated number more visible.

I believe my bad eyes and bad light conditions of my office are not that
unique, so making the generated keys more visible could be useful.

And yes, 'font-family: monospace' is rather brutal thing, but I believe
it really helps
(http://www.joelonsoftware.com/uibook/chapters/fog0000000063.html)

Signed-off-by: Matěj Cepl <mcepl@redhat.com>