Saving the colour theme in the theme settings creates an error.

I created the attached patch based on the report but haven't tested it myself.

Update the patch. please Review

Im am still having this problem.

Directory permissions are okay, in a subtheme which is newly generated. PHP max_input_vars = 10000

Brand new Drupal 10 install. Don't know where else to check.

I think I found a solution for this issue some months ago:

In the file at_core/forms/color/color_submit.php change the line 98 from

$paths['source'] = \Drupal::service('extension.list.theme')->getPath($theme); to
$paths['source'] = \Drupal::service('extension.list.theme')->getPath($theme) . '/';

I don't know if it will solve your problem but it solved mine.
Good luck