If you could please lodge a ticket saying attention to me, with some details, ie the name of the themes this happened for, I’ll look into it.
In general you shouldn’t have to change the design, but we do have a requirement that the theme should look reasonable and show content etc out of the box. Anyway, if I have some specifics, I can look into it.
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You don’t need to make the demo content editable in Gutenberg, but you need to load the front end styles so that if the user creates new content in Gutenberg, it looks like it will on the front end.
If you install the latest version of Twenty Seventeen and edit a post, you will notice that the content in the Gutenberg editor does not look like it does by default. Instead it looks like it will on the front end of the website. Headings are styled the same, the same fonts are used etc etc. Twenty Seventeen is loading the theme styles inside Gutenberg so it looks the same as the front end.
Eg: This is what Gutenberg normally looks like:
This is what it looks like in Twenty Seventeen:
Which is the same as the front end of Twenty Seventeen:
That’s what we mean by optimised for Gutenberg. If the user edits content in Gutenberg then it should look like the front end. Ie you need to style the editor content to match the frontend output as closely as possible, including any fonts used and any dynamic styles coming from settings etc.
Hope that makes better sense!