WordPress theme errors can result in error messages or blank white pages, which is commonly referred to as a white screen of death. You can use the steps below to diagnose potential WordPress theme errors.

Prepare your WordPress site for troubleshooting:

You should always backup your site before performing any troubleshooting.

Disable all plugins, especially any caching plugins.

If your site uses wp_cache, you will need to disable WP_CACHE.

Look for a theme related error

Enable WordPress debug mode and visit your site and check to see if there is a PHP error.

If there is a PHP error, check to see if the themes directory is in the file path. Below is an example:

        Fatal error:  Call to undefined function functionName() in
        /home/wp-content/themes/twentyeleven/functions.php on line 1

If the file path contains themes, then this confirms the issue is with the theme, and you can move on to fixing the error.

Determine if the error only occurs with your currently active theme

Connect to your hosting with FTP to upload files.

Navigate to the directory containing your WordPress site.

Then open the wp-content/themes directory.

Make note of the latest default theme that is installed.

Note: Default WordPress themes are named after the year they were released. For instance, the default theme that came out in 2017 is named twentyseventeen.

Update your WordPress site to use a default theme in the database.

Check your site. If the issue is no longer persisting, then you have successfully determined the issue is with your non-default