WordPress Classic Editor Font

If you want a font for the WordPress Classic Editor (TinyMCE) that isn't the Georgia Font (strikingly similar to the Microsoft Times New Roman font), then do the following;

• Create a file in the ./themes/ directory, name it anything.css (suggestion: tinymce_font.css) with the following code;

  body#tinymce.wp-editor {
  font-family: Arial, Helvetica, sans-serif !important;
  }

• Then add the following code to the theme's (parent or child) functions.php file;

function alternate_font()
     {
     add_editor_style( 'tinymce_font.css' );
     }

add_action( 'after_setup_theme', 'alternate_font' );

Source from: https://premium.wpmudev.org/forums/topic/make-my-default-fonts-as-arial-from-georgia-in-the-text-editor/