Bug 27566: Add generic font family to prevent unstyled select tags

Bug 27566: Add generic font family to prevent unstyled select tags

This patch adds a default font family "sans-serif" to the OPAC CSS as a
workaround for this Firefox bug:

font-family isn't honored in `<option>` element within `<select>`
dropdown
https://bugzilla.mozilla.org/show_bug.cgi?id=1536148

To test, apply the patch and rebuild the OPAC CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

 - Open the OPAC main page in Firefox.
 - Click the search type dropdown. The options should be styled using
   your system's default sans-serif font rather than a serif font.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Henry Bolshaw <bolshawh@parliament.uk>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>