Find the right selectors?

Hello,

I don’t understand how to find these selectors.

Where can I finf it?

I found it in my header.php

…
<nav id=”site-navigation” class=”main-navigation” role=”navigation”>
<?php wp_nav_menu( array( ‘theme_location’ => ‘primary’ ) ); ?>
</nav><!– #site-navigation –>
…

But I don’t know if it’s usefull ! If someone could give me a hand.

Thanks a lot.

My theme : https://ww.wp.xz.cn/themes/gravit/
My blog : http://www.funworldtour.com/

https://ww.wp.xz.cn/plugins/yith-infinite-scrolling/