webadmin: fixed contrast issue in search bar prefix label

PatternFly update introduced a change to the search bar, and the prefix label
was hard to read because it was gray on gray. This updates that label to be
black text in a light gray box, which is much easier to read.

Change-Id: I50cc91a5943f350291b5e1d8938a0912dbe30a7b
Signed-off-by: Greg Sheremeta <gshereme@redhat.com>