What is a theme builder?

Working with the Theme builder is an advanced feature which most users will probably skip.
Seeing as FormTitan offers ready made free themes, and that styles may also be changed inside the form builder, It makes the theme builder a feature for advanced users.

What exactly is the theme builder, and when should it be used?
While the form builder is mainly in charge of creating the form (dragging elements, configuring them...) the theme builder is only in charge of the styles given to the form elements (style of page, form, header, elements etc.)
To make it simpler you can compare the two entities to a book and its cover, a man and his cloths, a present and its wrapping...

as I said before, "the form builder is mainly in charge of creating the form". I say "mainly" because it can also be used to change the styles of the elements in the form.
If this is the case, then what do you need the theme builder for?

Yes- it is true that style changes maybe done equally in the form builder as well as in the theme builder, however there is a significant difference between the 2 options:
when you add styles to elements inside the form builder, they are applied on to the elements themselves, and become a part of them. this makes the styles harder to remove (since you need to remove each style from the element it is attached to).

When you create a theme and define the styles of elements inside the theme builder, you are actually constructing a style definition which is built
separated from the form and may later be applied to it, or to any other form. since it is a separate entity from the form it can easily be removed from it and replaced by another theme.