This is where we decide under which conditions the UI and the user CPT are available.

At #34885 (comment) I argued that I lean towards only enable them when the user has a theme.json, otherwise, we don't know whether the user styles are going to play well with the theme's. Riad brought up we should open this up for themes that have block templates but don't have theme.json as well.

As long as both places use the same logic, I'm not strongly opinionated. I went here with the current logic of the UI.

If we go with theme.json or block templates, I do wonder where's the logic we use to decide whether the edit-site is available. Shouldn't we tie this into that?

I've just checked and we don't load the site editor if the theme does not support block templates. So, if we want to update the logic, block templates are the only thing that matters to this as well.

Should it be be gutenberg_supports_block_templates or gutenberg_is_fse_theme. Meaning should classic themes that support block templates allowed to have global styles? Leaning towards no but curious what you think.

We enable the site editor only for themes that actually have templates (gutenberg_is_fse_theme), so this should be enough of a check to enable GS as well. As a curiosity, support for block-templates is added by us if the theme has a theme.json, so it's a redundant if we already checked for theme.json support.

However, note that this starts to get convoluted when/if we want embed GS in other screens (template or post editor). It's a lot simpler (and safer in terms of styling, in my view) if we only enable GS for themes with theme.json.

Ok, I pushed changes to make global styles (UI+CPT) use the same check than the site editor.

Ok so as it stands, this PR means Global Styles UI is only for FSE themes right? Meaning a classic theme with theme.json won't have access to it (if we potentially move the global styles UI elsewhere, say to the post editor or the customizer). That's fine by me for now but it also make me wonder why we need the check in the frontend since basically the site editor is not rendered in the exact same conditions.

Oh I just noticed that you removed the frontend checks nevermind

Right. To confirm: both site editor & GS are now shown under the same conditions with this PR (gutenberg_is_fse_theme).

I could have avoided this change by using gutenberg_is_fse_theme directly is GS code. However, it feels like if the edit site & GS are connected "conceptually" as we're arguing, they should also be explicitly connected by code. So, when/if our future selves want to update this logic is clear what it affects to.