CODE EXAMPLE FOR PHP

add custom style to wordpress editor

// ADD CSS on WP Editor
// On your functions.php file inside your theme
add_action( 'after_setup_theme', 'mam_gutenberg_css' );
function mam_gutenberg_css(){
  add_theme_support( 'editor-styles' ); // if you don't add this line, your stylesheet won't be added
  add_editor_style( 'custom-editor-style.css' );
}

// after create the 'custom-editor-style.css' file on the theme path and put whatever