Last East Rail train departs Lowu for East Tsim Sha Tsui at 12:30a. It'll be tight even if you take a taxi.

Arriving SZX this late, you need to use the 24-hour Huanggang crossing. Probably too late for the "direct" buses, but you can always take a taxi to Huanggang, cross the border, and then there are 24-hour bus connections to various points in Hong Kong.