You will notice that in the Look and Feel settings the Classic theme has a thumbnail preview of what it looks like, while our theme has only a broken image. To correct this, take a screenshot of your theme and save it in _diffs/images with the name thumbnail.png. It must have the exact size of 150 pixels wide by 120 pixels high. You should also save a larger version in the same directory with the name screenshot.png. Its size must be exactly 1080 pixels wide by 864 pixels high. After redeploying your theme, it will have a thumbnail preview just like the Classic theme.