You can write your email template outside FeedbackFive, then simply paste it into the FeedbackFive template editor. FeedbackFive will automatically convert it to plain text to avoid including extra styling that may be blocked by Amazon.
Amazon's Communication Guidelines do not allow sellers to send messages with font family, font color, or line height styling that would override Amazon's template styling. When you copy and paste text from somewhere else into the FeedbackFive template editor, all styling will be removed to keep your Seller Central account safe.
Then, you can re-add any styling that is allowed by using the options in the FeedbackFive template editor.