If you get the message Your site does not have a default theme, it is because you are missing the default theme. The default theme is used by WordPress in case anything goes wrong with your current theme.
If you delete the default theme, you will get the message Your site does not have a default theme. How to fix it? Just re-install any 20-series theme, like twenty twenty or any of the previous default themes.
In WordPress version, 5.2 WordPress suggested deleting all unused themes. As a result, many delete the twenty twenty theme or any previous default versions.
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