Hi!

The syntax in the /Configuration/RTE/Default.yaml created by Site Package Builder seems to be wrong (for TYPO3 13):

created, but not working (rte.css not loaded);

editor:
config:
contentsCss: "EXT:sitepackage/Resources/Public/Css/rte.css"

Working:

editor:
config:
contentsCss:
- 'EXT:sitepackage/Resources/Public/Css/rte.css'

(see https://docs.typo3.org/c/typo3/cms-rte-ckeditor/13.4/en-us/Configuration/Reference.html#editor-config-contentscss)

Best regards,
Christoph