Found a temporary solution, not sure if the postdivrich is the best choice but it works.
add_action('admin_head', 'my_custom_css');
function my_custom_css() {
echo '<style>
#postdivrich {
background-color:white; padding:10px 10px 10px 10px;
font-size: 16px; line-height: 1.5em; font-weight: bold;
}
#postdivrich::before {
content: "Post Main Body";
}
</style>';
}