To remove a theme from the website follow the steps mentioned below :
1. In your Shopify admin panel go to the Online Store ➜ Themes section :
2. Find the theme you want to remove and click on the Actions on the right side of it.
3. Select Remove from the drop-down menu of the action and confirm the deleting.
Your selected themes will be deleted.
Hope this tutorial will be helpful to you!
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Shopify Theme Options Overview
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Add An Admin User And Set Permissions To It | Shopify Themes