Wie kann ich Eingabefelder auf gleiche Breite einstellen?

You should first get the ID of the <li> element, then you need to copy, here is an image for reference: 

Then once you've copied the ID, replace it on the next CSS code: 

li#copiedID{

    margin-left: 10px;

}

This will help you to move the element, you can add more CSS rules so you can customize the field. 

This guide will help you to inject the code toy our form: How-to-Inject-Custom-CSS-Codes

You should do the same for all the elements that you want to customize on your form.