Add basic support of interactive themes ('theme' property in JSON)

[#84992110]
Tweak Lab styles so custom themes are easier to write

[#84992110]

- add border around the whole interactive, not only top and bottom bars
- slider background independent of the default Lab bg color
- remove CSS rules affecting html, body and other elements that
  are *not* inside .lab-interactive-container scope
- specify font-family per lab-interactive-container, not html (see above)
- remove or rename some other styles