Core themes throwing stylesheet was not properly added error.

[gwwar]

Description

I'm not sure if this is expected behavior, but when inserting certain types of blocks we can see a "stylesheet was not properly added error" in console.

Edit: So chatting with folks we do still support these themes, the console error is more of a warning to use a newer path.

Let's change the console.error to console.warn to make this less scary (deprecations typically show up as warnings vs error).

Step-by-step reproduction instructions

1. Use one of the following themes (twentyten, twentyeleven, twentytwelve, twentythirteen, twentyfourteen, twentyfifteen, twentysixteen, twentyseventeen, and twentytwenty)
2. Add a block that has a styles block setting panel (image block, site-logo, social-links etc)
3. Make sure the sidebar is open and select the block
   Expected: no js errors thrown
   Actual:

Screenshots, screen recording, code snippet

CleanShot.2021-09-21.at.10.55.38.mp4

Environment info

Gutenberg trunk / wp-develop master

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[gwwar]

Going to refile this in trac since we'll need theme updates

[gwwar]

Added https://core.trac.wordpress.org/ticket/54158#ticket

[gwwar]

So chatting with folks we do still support these themes, the console error is more of a warning to use a newer path.

With that in mind I'll update the issue to change the console.error to console.warn to make this less scary (deprecations typically show up as warnings vs error).

[oandregal]

errors have been transformed into warnings at #35914 so I presume we can close this now.