1. Go to Storefront › My Themes.
2. Click the Upload Theme button by the My Themes heading below the Current Theme.
3. Drag the ZIP file into the Upload Theme pop-up window, or you can Select theme from your computer.
4. Once the ZIP file has uploaded, a theme card will appear under My Themes showing theme processing progress. Click Cancel if you want to halt processing and start over.
5. After processing, a new custom theme card will appear under My Themes. If your theme was unable to be successfully processed, you will be presented with a screen detailing any errors encountered. Ensure your theme meets all the custom theme upload requirements.
Quick video Guide – how to upload bigcommerce stencil lilium theme.