selector:before{
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: url('URL DA IMAGEM AQUI');
    animation: rain 0.2s linear infinite;
    opacity: 0.2;
}

@keyframes rain{
    0%{
        background-position: 0 0;
    }
    
    100%{
        background-position: 100% 100%;
    }
}

selector{
    position: relative;
    animation: lightning 4s ease-out infinite;
}

@keyframes lightning{
    0%{
        filter: brightness(1);
    }
    
    86%{
        filter: brightness(1);
    }
    
    87.75%{
        filter: brightness(1.8);
    }
    
    89.5%{
        filter: brightness(1.3);
    }
    
    93%{
        filter: brightness(3);
    }
    
    100%{
        filter: brightness(1);
    }
}