The email templates for the Subscription app can be found at:
/plugins/j2store/app_subscriptionproduct/app_subscriptionproduct/mailtemplates
You can customize these templates by writing a template override.
Here are the instructions
Copy the following folder
/plugins/j2store/app_subscriptionproduct/app_subscriptionproduct/mailtemplates
\to
/templates/<YOUR_TEMPLATE>/html/plugins/j2store/app_subscriptionproduct/mailtemplates
Now you can edit each template and add your own custom wordings and styles
Example:
Edit
/templates/<YOUR_TEMPLATE>/html/plugins/j2store/app_subscriptionproduct/mailtemplates/notify_expire.php
Make the changes and save.