Rich text elements style missing after getting text from CMS


I’m designing my blog using CMS. And I have a Rich text element on the blog post templet. I made some style change and added images to the rich text element, then I click “get text from CMS blog posts - post body (rich text)”. It applied perfectly. But then I decided to change some style and uncheck the option “get text from CMS blog posts”, the rich text element went back to the default look. The changes I made earlier were missing (the image, new heading style,etc.) What should I do now? How can I modify rich text styles on a CMS templet if I already “get text from CMS”?
I made a screen record of what I did.


Here is my public share link: LINK
(how to access public share link)