That’s 3 questions!
1. Over time, the wood board of an mbira becomes darker from exposure to light, the natural oils on the hands of the musician, and sometimes dust/dirt. The keys become darker due to corrosion, and the different types of steel used by different makers vary in how quickly this happens.
2. It is important NOT to oil the board of an mbira, which could make the keys pop out when played, instead of staying in place. No routine upkeep is needed other than occasional tuning and tightening of keys. But some musicians use Vaseline on their thumbs/finger when playing, which also polishes the playing end of the keys, making them smoother and less likely to cause blisters when playing all night. Others may rub thumbs on their nose or forehead in order to use natural oils for the same purpose. Mbiras sold at mbira.org have Vaseline rubbed on the keys below the crossbars to prevent corrosion while stored in a coastal environment in California. Again VERY important NOT to put Vaseline on the keys at or above the crossbars, as it might leak down to the wood and result in keys popping out.
3. Mbira players typically travel with the mbira in a deze, which also protects it from the elements. However, in 1974 I once saw Ephat Mujuru scold a member of his mbira group who had his mbira in a clamp on the back of his bicycle, exposed to the rain which was starting.
For those who haven’t seen an older mbira yet, these are the mbiras of the Dzapasi Mbira Group.