Syling Custom Fields echo’s from from functions.php

How about just echoing out the needed syntax like that:

echo '<div class="custom-field">' . genesis_get_custom_field('instrument') . '</div>';

You can also just use CSS and style the

header.entry-header {
    //your sexy styles goes here
}

Since other elements inside are wrapped in their own elements and styled separately (I mean the actual H1 and meta data) the only altered item will be the bare custom field value.