But when you go Themes file, you can only find Add New Theme, but no sign of Delete.
Besides keeping the present active theme which you are using, you must also need to have a default theme for possible troubleshooting at any time.
When you are troubleshooting an issue, you can switch to these theme temporarily to confirm or eliminate your theme code as the cause of the problem.
You need to keep as least a Twenty Thirteen and Twenty Fourteen themes in your dashboard.
In fact you can also see all your active together with unwanted themes in the root directory at your host server (Blue Host, HostGator, or GoDaddy).
But when you delete those unwanted themes from your WordPress Dashboard, it will automatically be removed from your server as well.
So, you DO NOT need to go to your host server to delete them.
How To Delete Unwanted Themes In WordPress Dashboard
1. Log in to your WordPress admin area.
2. Scroll down and click on Appearance.
3. Select Themes and click on it.
4. Now you can see all themes there.
5. Hover your cursor (mouse pointer) over the themes which you like to delete.
6. The theme will turn blur and a “Theme Details” button will appear as shown below:
7. Click on the button "Theme Details". A page with the selected theme will appear with three buttons labelled Active, Live preview and Delete respectively.
8. You click on the Delete button as shown below:
9. A box will pop up asking: "Are you sure you want to delete this theme?" as shown below:
10. Click the OK button.
That's how you remove or delete your unwanted or unused themes in your WordPress Dashboard.