I'd like to override the default page blueprint from a theme. I tried to add the following to themes/mytheme/blueprints/default.yaml:

---yaml
'@extends':
type: default
context: blueprints://pages

form:
fields:
tabs:
type: tabs
fields:
customtab:
type: tab
title: Custom

[...]

However the tab doesn't show up. Do I need to create a blueprint for every page type to inherit from my modified default blueprint?
---yaml
'@extends':

Try extending parent@ as exampled here: https://learn.getgrav.org/forms/blueprints/advanced-features#extending-base-type-extends

Thanks, I tried this but it didn't work. It might be that the default.yml in my theme is not part of Grav\Common\Page\Types::systemBlueprints and does not get picked up as a fallback in scanTemplates() later on.

Make sure you call it default.yaml, not default.yml

Oh, I meant to write default.yaml (the file was named correctly).