1) What is it meant by player customizations? What exactly can be changed through the UI and what not?
2) For those items that can not be customized through the UI, how can they be customized?
1) The ThemeBuilder or Backlot based player customizations like color, highlight, pause screen info etc work only with the Flash Player. The settings are ignored by the HTML5 player.
The playlist or pods customizations via Themebuilder are all Html5 based settings so playlist customizations work when associated with flash or html5 player.
2) For any advanced customizations that is lacking in ThemeBuilder or Backlot, customers can use JS/CSS based page level customizations for Flash and HTML5 Players.
You can see full details on player customizations in the ThemeBuilder section of our documentation. As mentioned earlier, player level customizations apply only to Flash Player and playlist or pod level customizations can be used for Flash and HTML5 players.