Code for Logo Only

gistfile1.txt

<?php
add_action( 'amp_post_template_css', 'xyz_amp_additional_css_styles' );

function xyz_amp_additional_css_styles( $amp_template ) {
    // only CSS here please...
    ?>
    nav.amp-wp-title-bar {
        padding: 12px 0;
        background: #000;
    }
    nav.amp-wp-title-bar a {
        background-image: url( 'https://example.com/path/to/logo.png' );
        background-repeat: no-repeat;
        background-size: contain;
        display: block;
        height: 28px;
        width: 94px;
        margin: 0 auto;
        text-indent: -9999px;
    }
    <?php
}?>