Apple Pages underline
Pages does not expose this control and from experimentation using large font face sizing (96 and above), it appears the spacing between the underline and the characters varies on the typeface selected.

For example - see Courier and Courier New - the second font has a much larger spacing than the first.

It looks like you'd need a tool to edit the typeface or choose a typeface that matches your expectations. It is a bit surprising now that I look how close the underline hews to the characters for Pages default styles.