When creating a custom Drupal theme, you can encounter the following error message:
Drupal\Core\Extension\InfoParserException: Missing required key ("base theme") in themes/custom/mytheme/mytheme.info.yml, see https://www.drupal.org/node/3066038 in Drupal\Core\Extension\ThemeExtensionList->createExtensionInfo() (line 269 of core/lib/Drupal/Core/Extension/ThemeExtensionList.php).
Solution
The solution is to add a base theme key:
base theme: stable
"... we now require the base theme to be explicitly specified.", more on https://www.drupal.org/node/3066038
Full example:
name: My theme
type: theme
base theme: stable
description: 'Custom theme MyTheme'
package: Other
core_version_requirement: ^9
regions:
header: 'Header'
pre_content: 'Pre-content'
breadcrumb: Breadcrumb
highlighted: Highlighted
help: Help
content: Content
page_top: 'Page top'
page_bottom: 'Page bottom'
sidebar_first: 'First sidebar'