In the Look and Feel settings, the Classic theme has a thumbnail preview image, but our theme has only a broken image. To remedy this, create a PNG file named screenshot.png and place it in your theme’s docroot/_diffs/images directory. The image must be precisely 1080 pixels wide by 864 pixels high. On redeployment, the thumbnail will be automatically generated from screenshot.png.
Now go to the Look and Feel settings. Your theme’s thumbnail should appear there, along with the Classic theme’s thumbnail.
Tip: To achieve the 1080x864 resolution in a new browser window, use the button provided in the Resize Browser web content article displayed on the Welcome page.