BookStack/resources/views/common/custom-styles.blade.php

<style id="custom-styles"
       data-color="{{ setting('app-color') }}"
       data-color-dark="{{ setting('app-color-dark') }}"
       data-color-light="{{ setting('app-color-light') }}"
       data-color-light-dark="{{ setting('app-color-light-dark') }}">

    @php
        $settingSuffix = setting()->getForCurrentUser('dark-mode-enabled') ? '-dark' : '';
    @endphp

    :root {
        --color-primary: {{ setting('app-color' . $settingSuffix) }};
        --color-primary-light: {{ setting('app-color-light' . $settingSuffix) }};
        --color-link: {{ setting('link-color' . $settingSuffix) }};
        --color-bookshelf: {{ setting('bookshelf-color' . $settingSuffix)}};
        --color-book: {{ setting('book-color' . $settingSuffix)}};
        --color-chapter: {{ setting('chapter-color' . $settingSuffix)}};
        --color-page: {{ setting('page-color' . $settingSuffix)}};
        --color-page-draft: {{ setting('page-draft-color' . $settingSuffix)}};
    }
</style>