Here is how it works:

Create your form. When finished go to the 'Form widget' menu. 

Open 'Design' tab to see all of the Templates available as shown below:

Here you can edit:

Colors (Background, Widget background, Text, Highlighted blocks, 'Section heading') and background image

Font

Form name and description (Text, Color, Size)

Labels and Tooltips placement

Borders (Color, Width, Radius)

Upload and edit Logo

Submit button

Form width

Upload your CSS-file

Iframe settings

You can also edit theme. Read how to select theme for Table widget here.