When a theme is already active and a user tries to activate another theme,
the new theme configuration should be written to the registry so that it's
still in effect after a wine reboot.

Fix a regression introduced in d290362c.
Signed-off-by: Zhiyi Zhang <zzhang@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>