It is described in the message of shanna.
https://forum.emclient.com/emclient/topics/themes-in-em-client
"Bryan, you can add a thumbnail to a theme by taking a screenshot, using the top left portion, and reducing it to 80x80. Save it as a 24-bit PNG. Run it through https://tinypng.com/ , then encode it as base 64 data using this link
http://www.askapache.com/online-tools/base64-image-converter/
Then take only the data portion (after the comma) and put it into the
base64 data here
tag at the bottom of the theme file."