TinyMCE Image Options by Arne

content_style

This option allows custom CSS styles to be set as a string. The styles are injected into the head of the page containing the editable area. In TinyMCE’s classic mode, it is injected into the head of TinyMCE’s iframe. In inline mode, it is injected into the head of the page TinyMCE is rendered in.

content_style styles are not saved within TinyMCE’s content. If they are needed for display purposes, ensure the styles are also included in the page the content will be displayed on.

Example: Applying one CSS style using content_style

tinymce.init({
 selector: 'textarea', // change this value according to your HTML
 content_style: 'div { margin: 10px; border: 5px solid red; padding: 3px; }'
});

Source: Adding or changing the editor content CSS