When you look at the theme list in the Dashboard of WordPress, you see there is not a method shown to delete unwanted themes. So how can you remove them?
First we should ask the question ‘Why would you want to remove them?’ Well, they do not affect the speed of the site to any great degree. However, removing them does reduce the size of any theme included backup you may take, which could make a major difference over time. It also makes a more tidy site. So, removing unsed themes is probably a good move.
Remember NOT to remove any parent (main) theme from which you intend to use a child theme.
There is a way to remove unused themes, but it is a little hidden from first view. The Answer is to click on the themes for move information. In the bottom right hand corner you will find a delete button. You need to click and confirm and the theme is deleted.