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

<style>
    header, #back-to-top, .primary-background {
        background-color: {{ setting('app-color') }} !important;
    }
    .faded-small, .primary-background-light {
        background-color: {{ setting('app-color-light') }};
    }
    .button-base, .button, input[type="button"], input[type="submit"] {
        background-color: {{ setting('app-color') }};
    }
    .button-base:hover, .button:hover, input[type="button"]:hover, input[type="submit"]:hover, .button:focus {
        background-color: {{ setting('app-color') }};
    }
    .nav-tabs a.selected, .nav-tabs .tab-item.selected {
        border-bottom-color: {{ setting('app-color') }};
    }
    p.primary:hover, p .primary:hover, span.primary:hover, .text-primary:hover, a, a:hover, a:focus, .text-button, .text-button:hover, .text-button:focus {
        color: {{ setting('app-color') }};
    }
</style>