It’s easy to amass unused themes in your WordPress install. Each year WordPress releases a new theme, and you get it automatically when you update WordPress. Additionally, there are the themes you used to use, the themes you tried but abandoned, and so on. So, you may ask, what’s the harm?
There are a couple of reasons why you don’t want to keep unused themes around, cluttering up your WordPress install.
1) Themes are subject to vulnerabilities. If the theme developer is NOT issuing updates, your site is vulnerable. If the theme developer is providing security updates, then you have to keep the theme updated to be safe. Updating unused themes is a waste of keystrokes, so I like to remove them instead.
2) Themes take disk space (which may or may not be an issue) but just by being there, they will slow down your backup routine and INCREASE the size of your backup file. Not good.
So, now you have two good reasons to delete unused themes. But here’s why I never delete ALL my unused themes, and always keep the newest WordPress-developed theme around, even if I’m not using it: troubleshooting.
One way to troubleshooting pesky WordPress problems is to switch themes, and see if the problem goes away. If you don’t have an extra unused theme all ready installed, you now have to install one. Depending on the problem you are having, installing a new theme may be difficult. (FYI, another troubleshooting tip is to deactivate all your plugins. But I digress.)
So, now you know why to delete all but one extra theme. Deleting them is easy. Here’s how.
Go to Appearance / Themes and click on the theme thumbnail. When the Theme Details popover appears, look in the lower-right hand corner for Delete (it’s in red).
IMPORTANT NOTES: If your active theme is a child theme that requires a parent theme, the parent theme will NOT display a Delete button. Also, your active theme will never display a Delete button. Just like plugins, in order to delete a theme, it must be inactive.