The opposite is to 'board a bus'. 'Board' and 'alight' are the formal versions of 'get on' and 'get off'.
It's also worth noting that we never use the word 'alight' in everyday contexts. It's one of those words that you read in official notices in stations and hear in official announcements, but you never come across it in normal English. We just say 'get on' and 'get off'.