Here’s how to remove a theme from WordPress:
First: Backup
Before you do anything, make sure you backup your WordPress website so you can revert to the current version of your website should something go wrong. With that said, here we go:
How to Uninstall a WordPress Theme

First, go to Appearance / Themes

Hover over the theme you want to remove and click Theme Details

You’ll see a popup with the theme description.

Click on the Delete button

Confirm by clicking OK
That’s it, the theme is deleted.
Deleting an Active Theme in WordPress

Hover over the active theme and click on Theme Details

You’ll see that there is no way to remove the active theme

Go back to one of the inactive themes and click on Theme Details

Then click on the Active button to make this inactive theme active

The formerly active theme is now inactive and you can click on Theme Details to get it deleted

Finish it off by clicking on the Delete button