Custom theme throws “Undefined constant "CSS_STYLE" in constant”

Question

I just built a custom theme for Drupal 10. When enabling it via /admin/appearance, the dreaded message “The website encountered an unexpected error. Try again later.“ appears. drush watchdog:show says:

Error: Undefined constant "CSS_STYLE" in constant() 
(line 184 of web/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php)

My theme’s machine name is neinundamen. This is the neinundamen.info.yml file:

name: neinundamen
type: theme
'base theme': drupal_theme_letztegeneration
description: Subtheme of drupal_theme_letztegeneration
package: Custom
core_version_requirement: ^10
libraries:
  - neinundamen/global
regions:
  - ...

Here is the neinundamen.libraries.yml file, where I suspect my mistake - but I cannot see a problem.

global:
  css:
    style:
      css/style.css: {}

I tried the usual Drupal steps: updating to the most recent core version, clearing the cache using drush cr, drinking coffee.

Answer 1

You used the key style, but that is not part of the 5 allowed keys. Replace style: with theme: and you should be fine.

From the documentation:

You can set CSS weights with 5 different levels of styling:

1. base: CSS reset/normalize plus HTML element styling. Key assigns a weight of CSS_BASE = -200
2. layout: macro arrangement of a web page, including any grid systems. Key assigns a weight of CSS_LAYOUT = -100
3. component: discrete, reusable UI elements. Key assigns a weight of CSS_COMPONENT = 0
4. state: styles that deal with client-side changes to components. Key assigns a weight of CSS_STATE = 100
5. theme: purely visual styling (“look-and-feel”) for a component. Key assigns a weight of CSS_THEME = 200

These 5 levels are defined by the SMACSS standard.