PenciDesign replied
Hi,
1/ For change template for a post, you can go to edit that post > scroll down and you can see option "Select Single Style for This Post?" > and you can change to another post template there.
2/ For custom the content, the font, size...etc.. I recommend you use plugin https://wordpress.org/plugins/tinymce-advanced/ to add more those options for your blocks ( let's check on plugin description to know how to enable it )
Best Regards,
PenciDesign