LP#1755896: Web Client mobile menu font color contrast.

LP#1755896: Web Client mobile menu font color contrast.

When viewing the main menu in the web client in a small device or in a
browser with a width that is resized to less than 800px the menu
collapses and the submenu choices are difficult to read because of lack
of contrast.  This patch adds some contrast.

To test:
1. View the web client in a browser and lessen the width until the menu
   collapses, or view it in a small device.
2. Open some of the menu options to observe the submenus.
3. Apply the patch.
4. Refresh the browser and repeat steps 1-2 to view the more readable
   submenus.

Signed-off-by: Garry Collum <gcollum@gmail.com>
Signed-off-by: Terran McCanna <tmccanna@georgialibraries.org>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>