bug 5035 remove default opaccolorstylesheet value for new installs

bug 5035 remove default opaccolorstylesheet value for new installs

Per the bug, the color.css file is blank and so this preference
should have no value by default. This patch only fixes this for
new installs. There is no way to know if people have added values
to the color.css file in existing installs, so removing that value
in those cases might break things.

This patch does not remove the blank color.css file, it only
edits the default system preference.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>