there are a number of the form elements types that are still set to Pixel based measurements.
For example, In the style Editor, Select Form Elements -> Text
In the Size Section, there are settings for X-Large, Large, Medium, Small, X-Small. You will need to edit all of the available sizes, same for the other element types, there are mutliple sizes for each element that can be defined.
In reality, form builder does not have any stock designs that are based on fluid layouts. We will be adding this in future updates, but it is not included at this time.