Am I right that by default the themes are not kept in sync with their new versions?

Hi, I’m using the latest Ghost Docker image. I’d like to know if I’m thinking the right way that I will need to refresh the theme manually or with some custom integration if a new version of the theme rolls out? So at least no surprising refreshes will happen that would wipe out the modifications I made…

Correct, the only theme that is updated “automatically” is the default casper theme that’s included with Ghost. Any custom theme that is installed, even if they are based on Casper, will be untouched until you manually upload a new version of the theme.