In WSI you can edit any visual aspect of the plugin and email templates. You will be able to add your own branding or design very easily.
Just copy the templates folder located inside the plugin folder wordpress-social-invitations/templates and paste it on your theme’s folder.
Once you already copied it into your theme directory, rename the folder to “wordpress-social-invitations”.
So for example if you are using Twentytwelve theme, you should end having this file structure:
If you want to disable html template because you are using your own template system you can do it by simple adding the following line to wp-config.php
/** Disable html email from WSI**/ define('WSI_DISABLE_HTML', true);
This will simple return the Message text.