Themes are used to change the colors on the form. The Theme for the form can be selected in the Layouts and Theme section of the NITRO Forms Designer.
The Global Settings tab of the Forms Designer allows you to create new themes. Also, note that the Custom CSS section of the NITRO Forms Designer can be used to further modify the layout for a particular form.
Creating a New Theme
When in Global Settings, click the "Create Theme" button. This brings up the Configure Forms Theme box. This gives a large degree of control over what the form will look like. There are five tabs at the top of this window: General, Tab/Section, Controls, Buttons, and Search Dialog. Each has a variety of settings - see topics below. For in-depth discussion of the theme possibilities, please contact us.
Tabs in Configure Forms Theme -- see each topic for details
Editing, Deleting, or Copying a Theme
Any theme that users create can be edited. The themes that come with NITRO Forms Designer cannot be edited or deleted, but they can be copied into user-defined themes and then edited there. You can see the edit (pencil), delete (X), and copy (double sheets) options in the image below.
Refer below links for details:
Export / Import Themes In NITRO Forms
Using Themes To Enhance The UI In NITRO Forms