Lighten UI

- Use lighter background colors and borders to fit the rest of the
  Theme Customizer
- Add classes "button display-options" to Randomize button
Don't hide selected headers, highlight them

- ImageModel: hidden -> selected
- Remove old code (JS, CSS)
- Fix CurrentView's getHeight for "No image set" view
- Fix CSS for text placeholders using line-height
- Add blue border to selected header