Error to use base theme: classy

I got the same error. I have tried to enable classy theme but I cannot see in the theme list.

#3 yes we hope it will be solved soon

Did you enable the theme? It has to be enabled in the UI or via Drush.

Hi,
Classy used to be one of the core themes but now it has been removed and deprecated from core theme as of D10 and became a contrib one and unfortunately deprecated core themes don't appear in the appearance section. To make it appear in the appearance section you've to alter the info.yml file of the classy theme a lilttle bit. Change the hidden: true to false and it'll appear on that section.

Before creating sub theme of classy, make sure you install and enable the theme, you can do it from the UI or use drush command drush then to enable the theme. Without enabling the base theme you can't create sub theme of it!

And if you're creating a sub theme keeping classy as base theme you've to add base theme: classy and if you're defining your own library in libraries.yml make sure you attach the library properly to see its effect. To attach the library globally you've to add the library in your custom theme's info.yml libraries key. For attaching global styling use the key global-styling and to attach global scripts use global-scripts key in your libraries.yml.

Duplicate of 🐛 Remove "hidden" indicator from info.yml file RTBC , which already has a fix.