Hi,

Please go to Appearance → Customize → Custom CSS then add the following custom CSS code:

@keyframes mobilesticky {
    from {
        transform: translateY(-50px);
        opacity: 0;
    }
    to {
        transform: translateY(0);
        opacity: 1;
    }
}
.penci_navbar_mobile.mobile-sticky {
    animation-duration: 0.3s;
    animation-name: mobilesticky;
    animation-iteration-count: 1;
}

Regards,
PenciDesign.