silverstripe-framework/docs/en/02_Developer_Guides/15_Customising_the_Admin_Interface/05_Typography.md

title	summary	icon
WYSIWYG Styles	Add custom CSS properties to the rich-text editor.	text-width

WYSIWYG Styles

Silverstripe CMS 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 app/ folder. This is set through yaml config:

---
name: MyCSS
---
SilverStripe\Forms\HTMLEditor\TinyMCEConfig:
  editor_css:
    - 'app/css/editor.css'

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

Alternatively, you can set this on a specific TinyMCEConfig instance via setContentCSS method.

$config = new TinyMCEConfig();
$config->setContentCSS([ '/app/client/css/editor.css' ]);

Custom style dropdown

The custom style dropdown can be enabled via the importcss plugin bundled with admin module. (Doc)
Use the below code in app/_config.php:

use SilverStripe\Forms\HTMLEditor\TinyMCEConfig;

TinyMCEConfig::get('cms')
    ->addButtonsToLine(1, 'styleselect')
    ->setOption('importcss_append', true);

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;
}

Adding a tag to the selector will automatically wrap with this tag. For example :

h4.red {
    color: red;
}

will add an h4 tag to the selected block.

For further customisation, customize the style_formats option.
style_formats won't be applied if you do not enable importcss_append.
Here is a working example to get you started.   See related tinymce doc.

use SilverStripe\Forms\HTMLEditor\TinyMCEConfig;

$formats = [
    [ 'title' => 'Headings', 'items' => [
            ['title' => 'Heading 1', 'block' => 'h1' ],
            ['title' => 'Heading 2', 'block' => 'h2' ],
            ['title' => 'Heading 3', 'block' => 'h3' ],
            ['title' => 'Heading 4', 'block' => 'h4' ],
            ['title' => 'Heading 5', 'block' => 'h5' ],
            ['title' => 'Heading 6', 'block' => 'h6' ],
            [
                'title'           => 'Subtitle',
                'selector'        => 'p',
                'classes'         => 'title-sub',
            ],
        ]
    ],
    [
        'title' => 'Misc Styles', 'items' => [
            [
                'title' => 'Style 1',
                'selector' => 'ul',
                'classes' => 'style1',
                'wrapper' => true,
                'merge_siblings' => false,
            ],
            [
                'title' => 'Button red',
                'inline' => 'span',
                'classes' => 'btn-red',
                'merge_siblings' => true,
            ],
        ]
    ],
];

TinyMCEConfig::get('cms')
    ->addButtonsToLine(1, 'styleselect')
    ->setOptions([
        'importcss_append' => true,
        'style_formats' => $formats,
    ]);

API Documentation

• HtmlEditorConfig