On one of my apps, under the Styles menu, I’ve customized each element to my own design. I already have another app up and running with the default theme.
Right now the only way to copy a custom theme to another app is to copy & paste each style element over to the new app (reference)
It would be great to save our custom themes and sharing them across the apps (and even the community). This could even expand to a marketplace where users can sell their premium themes.