...
Plugins may set their own email templates. Take a look at each plugin specifically for more information on the email templates they support. For 3rd-party plugins, consult the 3rd-party plugins' documentation.
Styles
Email templates do not support embedded styles, e.g. within a <style></style> tag. Instead, styles should be inline.
For example:
.div { width:50px; } = BAD
<div style="width:50px;"> = GOOD
There is utility that may help convert your styles to inline styles at https://templates.mailchimp.com/resources/inline-css/