From time to time, you may come across some issues that may require you to revert your WordPress theme back to the original Twenty Seventeen theme without being able to access your wp-admin area. Below are a couple of ways to quickly switch to the default theme. As with anything, it is important to make sure that you're keeping frequent backups which is why iThemes Hosting provides you with access to BackupBuddy this way you can restore the backup should it be needed.
SFTP
This section does assume that you've already reviewed the SFTP article and understand how to use it.
To start off, you're going connect to your website using your FTP client. Once connected, you need to navigate to the web/content/wp-content/ folder. Inside the wp-content folder, you will find a folder called themes. This is where WordPress stores all of the themes installed on your website.
From here, you will want to download a copy of this folder to your local computer so that you can revert these changes if needed. Once you've got a copy of your themes folder, you will then want to delete the theme you're using. WordPress will automatically revert back to the default theme.
phpMyAdmin
This section does assume that you've already reviewed the phpMyAdmin article and understand how to use it.
You can also go into your phpMyAdmin and update the database's options table to reset your theme. To do this, access your phpMyAdmin, select your database and then click on the options table. (As you can see with this installation all tables in the database have the wp_ prefix before the table name. Your tables may have a different prefix.)
The following sections would have to be updated. You will likely need to click through a number of settings before you find all of the sections that need modified:
Template, stylesheet and current_theme.
You will choose the Edit option for each of these and set all of the values located at option_value to twentyseventeen.
You will then choose the Go option to save your changed settings. Then move on and do the same for each of the listed tables (template, stylesheet and current_theme).
Once you've changed all of the settings, check your website. If this clears up the whatever the issue was, then you should look at your themes functions.php file. There could be extra spaces at the bottom or you could be using a poorly coded function in your theme’s functions.php file.