• Inside your default template (eg. templates/templatename/css/), create a file name called editor.css, and add an import lines importing your current template’s css files.
    @import url("main.css");
    @import url("template.css");

    You may need to modify your body background if it's not white.

    body {background:#fff;}
  • Go to Extensions -> Plugin Manager -> Editor – TinyMCE, make sure Template CSS classes is on, then type editor.css on Custom CSS classes

Facebook Page