Within Wordpress, each blog post is a file, which Wordpress pulls into another file to display all of your posts on the page.
The reason you aren't able to edit blog posts with Semplice is that Wordpress does not offer that type of flexibility for themes within individual posts.
You are able to customize the theme of your blog/posts by navigating to Semplice > Advanced Styling > Blog Styling.
If you are familiar with code, you can heavily customize the blog to your liking. A good example is Tobias van Schneider's blog, which uses Semplice, with lots of customizations.
Troubleshooting: Why can't I edit blog posts with Semplice?
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