Update settings.php

[bookstack] / resources / views / partials / custom-styles.blade.php

<style id="custom-styles" data-color="{{ setting('app-color') }}" data-color-light="{{ setting('app-color-light') }}">
    :root {
        --color-primary: {{ setting('app-color') }};
        --color-primary-light: {{ setting('app-color-light') }};
    }
</style>