Enable themes to describe themeselves as admin or default in the info.yml file

Huge +1 for this. Thanks for creating the issue!

After the issues @lauriii linked, could we say this one and #3103375: Let themes indicate whether they work as front-end and/or admin themes could be merged? Maybe mark this one as duplicated and move all the info and relations to the previous one?

We now have three issues discussing roughly the same thing, so we should reconcile that.

I'm just going to quote my comment on #3103375-20: Let themes indicate whether they work as front-end and/or admin themes:

I think there is some value in guiding people towards making smart choices, in this case that is using the appropriate theme for the job.

However, I fully agree with @andrewmacpherson in comment 18 (#3103375-18: Let themes indicate whether they work as front-end and/or admin themes), there are very valid use-cases for using the admin theme as the front-end theme, headless Drupal or using Drupal as a business-process type of system are two very valid ones. I have more than one of those "business-process focused sites" that uses Gin as both the admin and front-end theme. Similarly, there are valid use-cases where you would want at least some of the administrative UI to use the front-end theme, maybe you have a community site with some users who have moderator level permissions, and you don't want them to have a jarring experience where they have to jump in and out of differently styled pages, I have a Drupal 7 site with that exact use case.

I think there are several completing concerns at play here:

1. For new users to Drupal, we want them to have a good experience learning and getting to know Drupal, so we should help them by guiding them to make smart choices about how they configure Drupal.
2. There are very valid use-cases for using the same theme as the admin and front-end theme (a headless installation, a business-process system, a community site, etc).
3. Theme developers may only want to support their theme being used in one particular context (front-end or admin), as to support the other context would require additional work and potentially additional user/community support which they are not able to provide.

I think we can find a way to reconcile those concerns; I can see the argument from all sides here, but above all else, I feel strongly that it is not the role of the theme developer to dictate in what way the theme should be used (again for all the valid reasons already mentioned), I think to do so would be incompatible with how we architect Drupal (Drupal is a framework that is not opinionated about how a site should function).

One of the ways we might be able to reconcile this is to provide the option for a theme to recommend how it should be used. For example, Olivero could indicate that it is meant to be used as a front-end theme (a key in the info.yml file probably). That recommendation would be communicated on the Appearance page, but if someone really wanted to, they would still have the option to set Olivero as the admin theme; They might be given a warning when doing so, something like "Olivero was not designed to be an admin theme, Olivero may not function well and support may be limited, proceed at your own risk.". Further to this, on the theme project page, we could provide an option for theme developers to recommend how their theme is meant to be used: as an admin theme, front-end theme, or both.

I think that would be a nice compromise: we effectively guide new users, we don't take the choice away, and we mitigate the likelihood of theme developers need to deal with issues/support that they didn't intend on (we give them something that they can easily point to).

We should not have 3 issues dealing with the same request. Let’s close this issue and use instead #550102.

@rkoller no problem at all! I couldn't find them neither even I was guessing something existed, so that's why I asked @lauriii. Thanks all for keeping the ball rolling.