It isn’t possible to setup a default stylesheet via appsettings unfortunately it has to be set up in the CMS as per Rich Text Editor Styles | Umbraco CMS
If your wanting to retain the nested list style for editors I found it easiest to use the style_formats app setting as per Rich text editor settings | Umbraco CMS.
This allows you to adds your default formats for your RTEs and you can also apply specific classes for different element types.
For example, this json (would need encoding) will add the headers along with a seperate nested list for custom list styles.
Thanks for the response @jawood1 just a follow up question on that, is it then possible to include the css/style in style_formats or can that work together with a stylesheet?We want to show the styling of how it would look like on the site
However, I can imagine if you have a lot it could get pretty horrible to maintain. Though, I think you could load them into the appsetting on startup as per this blog Formatting for RichTextEditor config
It is also possible to have an attached stylesheet as long as it doesn’t contain the /*umb_name:Test/ declarations against your elements it shouldn’t override the format set in your app settings.