The styling of most non-admin pages, such as the OpenID sign in pages, can by customised with CSS styles.

In System Preferences → Branding → Software you can select the active Style and edit Custom styles.

Styles

The selected Style adds a CSS class name brand-{styleCode} to the affected pages. The selected style can also be used to override translations. When a text on the page is looked up by its translationCode, a translation with code Style.{styleCode}.{translationCode} is looked up first. By adding such translations, a custom text can be used when the style is active.

Custom styles

In addition to the built-in styles, you can create new custom styles. Click the Custom styles button to open the Custom style management window.

Add, edit or remove styles in the window.

After creating a custom style you can select it as the active style.