That cured the white on black text in the editor, but I now notice the shadowed text & still white on black text in the preview mode. Solution? I have included snapshots to see the issues

The High Contrast Mode reset attempts to override some of the text styles sey by your template, so that text is readable in the editor, but it does not override text effects like text-shadow.

The Preview tab shows the editor content as it might appear in the front-end of your site, by styling the content with your tempate's stylesheets. This may not always be accurate, as the template could use multiple stylesheets that the editor is not configured to load, or different stylesheets for different parts of the site.

Which template are you using?

Please post a link to and article on your site.

Can you send me a login? -> https://www.joomlacontenteditor.net/contact/site-login

I have fixed this on your site by removing one of the template's unused compiled css files (whenever you make a change to the template, it creates a new compiled css file), which the editor was loading instead of the css file used by the template in the front-end.