Dialog

Drupal dialog uses jquery UI.

How do you override the default css?

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

https://git.drupalcode.org/project/drupal/-/blob/11.x/core/assets/vendor/jquery.ui/themes/base/theme.css?ref_type=heads

https://jqueryui.com/themeroller/