This issue is due to the theme directory or theme vendor name being renamed. Try the below steps to fix this.
1. Change your theme vendor first letter in capital.
2. Change the theme directory name into theme.
app\design\frontend\<Your-Vendor>\<theme-name>\theme.xml (if parent theme is not default magento)
3. Now, delete your current theme entry from the "theme" table, but keep back-up of this table before removing the custom theme entry.
4. Now run the command for setup-upgrade, static content deploy, and cache clear.
rm -rf var/di/* var/generation/* var/cache/* var/log/* var/page_cache/* var/session/* var/view_preprocessed/*
php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy -f
php bin/magento c:f && php bin/magento c:c
5. Change your store theme from admin.
If issue solved, Click Kudos & Accept as Solution.
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