Hi Fredrick,
We are using tinymce and it will need some custom code.

1. Please create a file bbl-editor in your child theme/theme and put some css code there like the following

body {
    font-size: 30px;
    color: green;
}

2. In your theme/child theme’s functions.php

// Add Tinymce css customization on front end.
add_filter( 'mce_css', function ( $css ) {
	if ( is_admin() ) {
		return $css;
	}
	$css .= ',' . get_stylesheet_directory_uri() . '/bbl-editor.css';

	return $css;
} );

That will allow you to customize as you need.

PS:- This will have effect on all of the tinymce instances on front end(in case you are using other plugins). If you only want to limit to buddyblog pro, Please let me know where the form is on a page or on profile and I will update it.

Regards
Brajesh

You are welcome.