Theme's yaml files have Yaml issues

[pamtbaau]

When installing theme Bootstap4 inside a site, the Blog and Item pages in Admin do not show the extra tabs as defined in blueprints blog.yaml and item.yaml.

When running $ bin/grav yamllinter multiple issues are shown.

This can be solved by adding the following to system.yaml

strict_mode:
  yaml_compat: true

This is covered when upgrading form Grav 1.6 to Grav 1.7, but not on fresh Grav 1.7 installation. Hence Yaml issues should be fixed in theme.