fix second level menu visibility as raised in issue 121
Fix 'Creating default object from empty value' warning.
Fix footer overlay.

When rendering an Infinity overlay option, set an additional CSS rule so
`display:block` applies to the :before pseudo element.

This ensures that overlays are applied correctly.

Fixes #182.
Port over latest bp-default strings to cbox-theme.

Props @btalberg.

Fixes #189.
fix z-index of dropdown submenus
When setting the background color, use the CSS "background" property.

Previously, when Infinity's "Background Color" theme option was set, the
CSS used the "background-color" property.

This makes logical sense, but when compiling the rest of the theme's CSS,
other background theme options could take precedence, thereby not rendering
the background color.

Therefore, this commit uses the CSS "background" shorthand property to render
the background color as this overrides various CSS background properties.

See #161, #88.
enqueue additonal bp scripts required by newer version