Ability to customize color of preview and facet panes, topic pane and right-hand menu
The ability to specify a background color, font, and font color for the preview, facet and topic panes.
As of the August release, you can now customize the background color for the facet pane.
Suzy Bailey commented
I'd like to see customization of the UI generally, but addressed in this way http://ideas.exlibrisgroup.com/forums/564566-summon/suggestions/19000279-add-a-custom-js-css-field-to-simplify-ui-customiza