`guide_custom()` does not have a `theme` argument.

Every guide has a `theme` argument to stylise individual guides.
The only exception to this should be `guide_none()`, but currently this argument is also missing in `guide_custom()`.
While there is not much to stylise, it *should* have a theme argument to control the display of the title and margins.
This is a small oversight on my part while implementing #5554.