e-newsletter custom theme creation help

I have a developer trying to help me with custom themes for the e-newsletter as the ones provided are very old fashioned.

I've purchased a theme pack for newsletters from themeforest and they look great but implementing isn't working out.

As adding only styles won't make much difference. We'll also need to add the actual template html file too somewhere else. The theme folder also doesn't includes any actual template file, which means all of these are being called from somewhere else where the actual templates (html files) are stored. This we are trying to understand.

My developer said: "I don't think we can add custom html code to the plugin. It would be better if you could contact the plugin developers. I've also contacted them via their website, waiting for an answer on this."

Hopefully we can get some movement on this as we have a large email audience for this site and others and would love to have some new templates to use.

Any help is greatly appreciated. Maybe a step by step guide of how to implement a new theme; or import a html theme?

Many thanks

James