Ckeditor5 in edit mode is loaded by js style, but does not provide in the display, the official provides two ways.
https://ckeditor.com/docs/ckeditor5/latest/builds/guides/integration/content-styles.html
By taking it directly from this guide and saving it as a static resource in your application (e.g. content-styles.css) (recommended).
By generating it using a dedicated script. Learn more in the Development environment guide.
Important!
If you take a closer look at the content styles, you may notice they are prefixed with the .ck-content class selector. This narrows their scope when used in CKEditor 5 so they do not affect the rest of the application. To use them in the front–end, you will have to add the ck-content CSS class to the container of your content. Otherwise the styles will not be applied.