Engage Box uses the Joomla MVC, that means you can copy any view or layout in your own template and personalize it as you want. Each box type (Custom Text, Email Subscription Form, Load Existing Module e.t.c.) is using its own separate layout file which can be overridden in your own template by just copying that file to your template folder.
The main benefit is that you will be able to update or upgrade it without losing your settings. Once you copied the file in your own template, you can modify the copied file as you want. Joomla will check if that file exists in your own template and load it, otherwise it will use the default file the extension's folder.
Override the main rstbox.php layout
Copy the file administrator/components/com_rstbox/layouts/rstbox.php in templates/YOUR_TEMPLATE/html/layouts/com_rstbox/rstbox.php
Override the Custom Text layout
Copy the file administrator/components/com_rstbox/layouts/custom.php in templates/YOUR_TEMPLATE/html/layouts/com_rstbox/custom.php
Override the Email Subscription Form layout
Copy the file administrator/components/com_rstbox/layouts/emailform.php in templates/YOUR_TEMPLATE/html/layouts/com_rstbox/emailform.php
Override the Load Existing Module layout
Copy the file administrator/components/com_rstbox/layouts/module.php in templates/YOUR_TEMPLATE/html/layouts/com_rstbox/module.php
Override the Social Media layout
Copy the file administrator/components/com_rstbox/layouts/social.php in templates/YOUR_TEMPLATE/html/layouts/com_rstbox/social.php
Override the IFrame layout
Copy the file administrator/components/com_rstbox/layouts/iframe.php in templates/YOUR_TEMPLATE/html/layouts/com_rstbox/iframe.php