SilverStripe lets you customise the style of content in the CMS. This is done by setting up a CSS file called editor.css in either your theme or in your mysite folder. This is set through

HtmlEditorConfig::get('cms')->setOption('content_css', project() . '/css/editor.css');

Will load the mysite/css/editor.css file.

If using this config option in mysite/_config.php, you will have to instead call:

HtmlEditorConfig::get('cms')->setOption('content_css', project() . '/css/editor.css');

Any CSS classes within this file will be automatically added to the WYSIWYG editors 'style' dropdown. For instance, to add the color 'red' as an option within the WYSIWYG add the following to the editor.css

.red {
    color: red;

After you have defined the editor.css make sure you clear your SilverStripe cache for it to take effect.

API Documentation

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