Copy the theme into the themes/ directory of your SilverStripe project. If you've named it correctly, there should be a directory called themes/simple/templates.

Add the following to your mysite/_config.php file. Remove any existing SSViewer::set_theme lines.

 SSViewer::set_theme("simple");
 HtmlEditorConfig::get('cms')->setOption('theme_advanced_styles', 'highlight=highlight;no-border=no-border,break=break');