A website I maintain uses a self-made theme based on a starter theme (Sage). It was developed in 2016, hence long before the block editor appeared. Now we want to go away (at least for some posts) from a page-builder to the WP block editor. I noticed that the recently introduced size settings (padding, margin), that are available for many blocks, are not shown in our installation. I did some tests and came to the conclusion that the theme is causing this. I already did some tweaks to the theme (e.g. making it PHP 8.0 ready), so I guess I can do it again. But I have no clue, how a theme can impact which settings are available for blocks or specif blocks. Can someone advice how to do it or provide a link to related documentation? Thank you!
You'll have to define the settings on theme.json. Here are the documentation links that would be of help.