To set a responsivity of the e-mail, click on the "Responsivity" icon.
A form will appear.
Select the "Common resolutions" option.
Set the breakpoint by sliding the navigator.
Select the "Custom resolution" option.
Insert the desired breakpoint.
To set custom styles, click on the "Custom styles for mobile display" text.
A text field will appear. Insert the custom styles.
Click on the "Confirm" button.