Off-Canvas Hover background: separate variable.

#Topic: Off-canvas list item background color, which applies when one hovers over it.

## Problem:
Hardcoded value.

## Solution:
Separate variable: $off-canvas-bg-hover

## Edited files:
* code: `scss/foundation/components/_offcanvas.scss`
* setting: `scss/foundation/_settings.scss`
* docs: `doc/includes/off_canvas/examples_offcanvas_variables.html`