Color contrast on autocomplete list focus does not meet accessibility

Color contrast when focussing on ui-autocomplete items needs more ctonrast

Steps to reproduce:

1. Type into linkit textbox with autocomplete suggestions
2. Use the down arrow to focus on one of the results. Text is #ffffff and background is #BFBFBF
3. Measure the color contrast as 1.83:1, which does not meet WCAG standards.

Proposed resolution

Change the background color to #595959 or similar in css/linkit.autocomplete.css by targeting the

.ui-autocomplete .linkit-result-line-wrapper.ui-menu-item-wrapper.ui-state-active,
.ui-autocomplete .linkit-result-line-wrapper.ui-menu-item-wrapper.ui-state-focus 

rule.