If you installed Messenger's official plugin for WordPress, you will see that after the initial setup it's not customizable... For example, I would like to put some bottom margin so that it doesn't overlap the footer's social icons, but there was no way of doing it through the plugin settings.
CSS is the Solution!
We open up CSS Customizer in WordPress and we add this lines
text
/* Facebook Chat Bubble */
.fb_customer_chat_bubble_pop_in {
margin-bottom:42px;
}
.fb_customer_chat_bounce_in_v2 {
margin-bottom:42px;
}
Now both the chat bubble and the inner content will be pushed up a notch ;)