Hello,
 
There are 2 ways you can add Cart to Header of any theme, including Seller.
First, use this plugin.
Second, insert this in an appropriate place in header.php where you want the icon to appear.
<?php if ( in_array( 'woocommerce/woocommerce.php', apply_filters( 'active_plugins', get_option( 'active_plugins' ) ) ) ) {
 
    $count = WC()->cart->cart_contents_count;
    ?><a class="cart-contents" href="<?php echo WC()->cart->get_cart_url(); ?>" title="<?php _e( 'View your shopping cart' ); ?>"><?php
    if ( $count > 0 ) {
        ?>
        <span class="cart-contents-count"><?php echo esc_html( $count ); ?></span>
        <?php
    }
        ?></a>
 
<?php } ?>

After adding the above code, go to functions.php and add the following at the end:
 

function themename_add_to_cart_fragment( $fragments) {
    ob_start();
    $count= WC()->cart->cart_contents_count;
    ?><a class="cart-contents"href="<?php echo WC()->cart->get_cart_url(); ?>"title="<?php _e( 'View your shopping cart' ); ?>"><?php
    if( $count> 0 ) {
        ?>
        <span class="cart-contents-count"><?php echoesc_html( $count); ?></span>
        <?php           
    }
        ?></a><?php
    $fragments['a.cart-contents'] = ob_get_clean();
    
    return$fragments;
}
add_filter( 'woocommerce_add_to_cart_fragments', 'themename_add_to_cart_fragment');

 
 
The above code will enable ajax refresh of cart contents in header. It’s not required, if you do not want it.