[Fix] Minor fix: check that CUs exist before determining main container
[Minor] Minor changes
[Fix] Fixed bug in _selectNext due to which a call was being made to isAnyPartOfCUinViewport() with a null argument

Mirroed the fix to _selectPrev which didn't have this issue because a recent commit which caused this issue
in _selectNext hadn't been mirrored to _selectPrev (in that commit we reversed the order of checking if a CU was
partially visible, followed by checking if it had been selected recently enough to be considered, since the former
is more expensive and should be made only if the latter fails)
[CSS] Minor: changed SASS variable name