Drupal 10. How to unset theme.css in custom theme?

@VasyOK,
It seems like you've made some indentation issue.
Proper indentation is crucial for the code to be parsed correctly.

Please follow-up with the below code, Hope it works.

Your code should look like this with correct indentation:

libraries-override:
jquery_ui/core:
css:
component:
assets/vendor/jquery.ui/themes/base/theme.css: false
assets/vendor/jquery.ui/themes/base/accordion.css: false

NOTE :-

In the above corrected code,
The component section is properly indented under css.
The CSS files you want to disable (theme.css and accordion.css) are correctly indented under component.
With this indentation, Drupal should be able to recognize and apply the libraries-override configuration as intended, disabling the specified CSS files from the jQuery UI library.

Sory abhiyanshu_rawat, it doesn't work.

Solution referensed from https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21...

add function to mytheme.theme

function mytheme_css_alter(&$css, \Drupal\Core\Asset\AttachedAssetsInterface $assets, \Drupal\Core\Language\LanguageInterface $language) {
  unset($css['modules/contrib/jquery_ui/assets/vendor/jquery.ui/themes/base/theme.css']);  
  unset($css['core/assets/vendor/jquery.ui/themes/base/theme.css']);
  unset($css['modules/contrib/jquery_ui/assets/vendor/jquery.ui/themes/base/accordion.css']);
}

Hi @VasyOk,
However, based on your reference,
Here's a more flexible and maintenance-friendly approach using the array_filter function to conditionally unset CSS files.

/**
 * Implements hook_css_alter().
 */
function mytheme_css_alter(&$css, \Drupal\Core\Asset\AttachedAssetsInterface $assets) {
  // List the CSS files you want to remove.
  $css_to_remove = [
    'modules/contrib/jquery_ui/assets/vendor/jquery.ui/themes/base/theme.css',
    'core/assets/vendor/jquery.ui/themes/base/theme.css',
    'modules/contrib/jquery_ui/assets/vendor/jquery.ui/themes/base/accordion.css',
  ];

  // Unset the specified CSS files.
  $css = array_filter($css, function ($key) use ($css_to_remove) {
    return !in_array($key, $css_to_remove);
  }, ARRAY_FILTER_USE_KEY);
}

It uses an array of file paths to remove and then filters the $css array to keep only the files that are not in the list of files to remove.

Make sure to clear the Drupal cache after adding or updating this code in your theme's mytheme.theme file.
The above code allows you to easily add or remove CSS files from the list as needed without altering the structure of the code.

Yes @abhiyanshu_rawat, you right!
Thanx!

@VasyOK,
I think we can close this issue now !!!

Automatically closed - issue fixed for 2 weeks with no activity.