How can I get the tagline under the logo instead?

Please try applying this custom CSS code by going to "Appearance > Customize > Additional CSS":

.tagline {
    position: absolute;
    bottom: -1.6em;
    text-align: center;
}
@media only screen and (max-width: 1165px) {
    .tagline {
        bottom: -1.6em;
        display: block;
    }
    .site-logo {
        position: relative;
    }
}
@media only screen and (max-width: 480px) {
    .tagline {
        width: 100%;
    }
    .site-logo {
        margin-bottom: 1.6em;
    }
}