Add Site, with Twenty Fifteen theme deleted, gives WSOD everywhere except Admin Panels

[adiant]

In a WordPress Network, Add Site creates each new Site with the Current Theme set to Twenty Fifteen, even when that Theme has been deleted (long before the Add Site was performed).

WSOD is displayed when viewing any "Front End" (non-Admin panel) URL, and no error messages are displayed, even with WP_DEBUG set to TRUE.

The solution is immediately obvious on the Appearance-Themes panel where 'ERROR: The theme directory "twentyfifteen" does not exist.' is displayed. Unfortunately, knowing to look there is not intuitively obvious.

[JPry]

You should be able to resolve this by setting the WP_DEFAULT_THEME constant in the wp-config.php flle to the theme you want to use instead. It will be the theme folder name.

If my theme folder is "jpry_theme", this is what I would set it to:

define( 'WP_DEFAULT_THEME', 'jpry_theme' );

For more information, see ​this codex article.

[adiant]

Since I support the right of the WordPress Development Team to decide whether any Issue is a Bug or Design Decision, I have re-opened this as a Enhancement, given how frequently I see WSOD errors when responding to Support forum queries. And also given the I.T. mindset that says "Delete all unnecessary Components to reduce Maintenance Costs", which is why it makes sense to a lot of people to delete the Twenty Fifteen theme, if they don't plan to use it.

I believe that any one (only need one of them) of the following enhancements would make this problem at least an Order of Magnitude easier to debug. Whenever the default Theme does not (or will not) exist, I am suggesting that any one of the following be implemented:
1) Error Message on "Front End" when WP_DEBUG set to TRUE, or
2) Warning on top of each Admin panel on New Site, or
3) Fatal Error when Creating a New Site, or
4) Fatal Error when trying to Delete "Default" Theme

Thank you for your interest in this. If it is decided not to proceed on this, I am willing to update the Codex to make it easier to find the solution.

[SergeyBiryukov]

Replying to adiant:

And also given the I.T. mindset that says "Delete all unnecessary Components to reduce Maintenance Costs", which is why it makes sense to a lot of people to delete the Twenty Fifteen theme, if they don't plan to use it.

I'd argue that removing default themes is a bad practice, since they are often an integral part of troubleshooting if something goes wrong with the current theme.

I am suggesting that any one of the following be implemented:
1) Error Message on "Front End" when WP_DEBUG set to TRUE, or
2) Warning on top of each Admin panel on New Site

See #21931.