Center the form

To center the form on the page, you have to specify a form width either in pixels (px) or percentage (%) in pixels, under form properties.
Go to form properties, as shown in the screenshot.
Then change the form "Width" property (by default it is "Automatic") to a fixed reasonable value, for example 400px.
Don't worry about responsiveness, the form is still responsive anyway. You are just going to set the maximum width of the form, but in smaller displays it will fit the available area.