Error with {{ page.highlighted }} while making a copy of Olivero

I am trying to learn Drupal 9 theming by copying Olivero theme into my themes folder.

I copied web/core/themes/olivero to web/themes/olive and then renamed all file names containing 'olivero' to replace it with 'olive'. Then I replaced all text inside all files from 'olivero' to 'olive'.

This gives me the error:

The website encountered an unexpected error. Please try again later. TypeError: Drupal\Core\Render\Renderer::doTrustedCallback(): Argument #1 ($callback) must be of type callable, array given, called in /var/www/html/web/core/lib/Drupal/Core/Render/Renderer.php on line 788 in Drupal\Core\Render\Renderer->doTrustedCallback() (line 51 of core/lib/Drupal/Core/Security/DoTrustedCallbackTrait.php).

I realized that the theme works if I remove {{ page.highlighted }} from the web/themes/olive/templates/layout/page.html.twig

Also, the theme seems to be working fine if I don't rename it and keep it olivero even if I move it to web/themes/

Can you please help me figure out how to fix this error and keep the {{ page.highlighted }} block?

Ok, I figured the issue out -- while renaming I failed to rename one file web/themes/olivero/src/OliveroPreRender.php because of the capitalization. Renaming the file + updating the class name fixes the problem.

You should not copy the theme to your local folder. If you want to modify a core theme, you should create a sub-theme of the theme you want to alter, and make your edits there. This means that any updates to the core module will flow down to your sub-theme.

Copying core themes and use it in themes/ is a wrong way in Drupal. Rather You should create a custom theme having a base theme Olivero.

You should follow this theme where I use Olivero as base theme: https://github.com/priyadegwekar/drupal9/tree/priya/web/themes/custom/olivero_sub_theme

With this you can keep highlighted region too.