keeweb/app/styles/themes/_flat-blue.scss

$themes: map-merge(
    $themes,
    (
        fb:
            map-merge(
                $theme-defaults,
                (
                    background-color: #282c34,
                    medium-color: #abb2bf,
                    text-color: #d7dae0,
                    action-color: #528bff,
                    error-color: #c34034
                )
            )
    )
);