I would suggest to ask any questions in the below custom themes thread. Lots of user help there.
https://forum.emclient.com/t/custom-em-client-themes/36440
Also if you are wanting custom themes, this is the user created eM CIient themes.
https://www.emclient.com/themes
Also eM CIient Themes online editor help
https://www.emclient.com/webdocumentation/en/9.2/emclient/default.htm#Settings/Themes.htm
Lastly eM CIient blog on designing your own theme.