If you do not have a default theme the Site Health check on the dashboard whines about it.
To get around that, download a default theme
Delete everything inside the theme except style.css
Edit the style.css and delete everything after the */ on line 21.
Now upload the theme folder with just that edited style.css in there to /wp-content/themes
And now the Site Health will not whine.
If/When the theme updates, just edit the version on line 4. Or be proactive and change the version now to 10.4