After the upgrade, The First thing you need to check the custom theme is applied correctly in the admin.
Below are the steps to check the selected theme in the admin.
In Admin, go to
Content > Design > Configuration. A Design Configuration page opens. It contains a grid with the available configuration scopes. For example:
Design Configuration page
In the configuration record corresponding to your store view, click Edit. The page with the design configuration for the selected scope opens. For example:
The design Configuration page for a particular scope
On the Default Theme tab, in the Applied Theme drop-down, select your newly created theme.
Click Save Configuration.
If caching is enabled, clear the cache.
php bin/magento c:c
If you still facing the issue then run the static content deploy command.
php bin/magento s:s:d -f
To see your changes applied, reload the storefront pages.