If the user just selects RGBA color values, size dimensions, etc., then mapping these user specifications onto a set of style attributes and HTML like
background-color: rgba(255, 0, 0, 1)
<button class=myButtonStyle>User wrote this text</button>
probably does not cause the program to generate novel copyrightable output beyond the user's own input.
Under the copyright-law concept of the merger doctrine, when a concept that is so simple it can only be expressed in a limited number of ways, those expressions are not copyrightable. There are a very limited number of ways to express a button in HTML, and even fewer ways to specify its color and size dimensions in CSS. The program's selection of
background-color: attribute names to prefix the user's input isn't a decision that is sufficiently creative to merit copyright protection over the program's contribution to the output. Thus, no license is necessary, because there is no copyright-protected expression to license to anyone.
If the program instead provides a sufficiently creative template file, with a structure complex enough to merit copyright protection, into which the user has simply dropped in a bit of text and images, then the resulting output might be a derivative of the program's original template. In that case, the program would need to specify how the resulting output may be modified and redistributed m (One expects the permissions granted will be fairly broad, or else not many people will choose to use the output of such a program -- it is for this exact reason the FSF discourages use of the GPL for template files.)