Update selected background on repeater, datepicker, and tree

Changed selected and hover colors to gray, (due to "light blue confusion"--myself included: whether a blue was Marketing Cloud theme or Bootstrap or Fuel UX, etc). Added variables for hover, selected and hover-selected to create consistency. See https://github.com/ExactTarget/fuelux-mctheme/issues/56

[This is @zerosquadron 's PR, but it had merge conflicts.]