Hi Marley, I've been meaning to make a custom themes video tutorial for awhile, and your comment pushed me to put it together some tutorial videos this weekend. There will probably be one more video added to this series, but hopefully this is enough to help you make your own theme.
Full playlist here: https://www.youtube.com/playlist?list=PLsyHcmEndd1Ue4EjqwfSnMMOhduuxIZvQ
Currently there's no way to add custom typing sound effects and soundscapes, but I will consider it for future versions.