We recommend using FTP to modify code in your theme. If you use Appearance > Editor, making a mistake is more likely and will produce a PHP error or white screen that stops your website from working.
How to bring your site back up ↑ Back to top
If you site has died, you need to use FTP or your hosts “File Manager” to access your theme files. They are stored in this folder:
Edit the file directly here, and undo any changes that brought the error in the first place. You can also rename your theme folder, which will cause WordPress to automatically activate the default theme.
Finding the error (white screen only)
If you don’t know what file is causing the error, you can enable WP_DEBUG in your wp-config.php which will tell you exactly which file is causing the error, and what line the bad code is on.
Customizing CSS ↑ Back to top
Customizing your CSS (e.g. custom.css) through Appearance > Editor is perfectly safe, as it will never produce any errors if you make a mistake. It is good to validated the CSS you add though, especially if it isn’t being applied properly. Use the Chrome DevTools to inspect your custom.css and see if the CSS is being applied correctly.