In RH2022 (Frameless) if you selected the "Tooltip" style, the popup is automatically generated by the browser from the content of the "title" html propery. So I don't think there's any way to customise it.
If you used the "Popup" style then you can customise it, although it doesn't look like Adobe allowed an easy way to do that (unless it's hiding in the templates somewhere). However, I poked around and you can try adding p.popover-text to your stylesheet, with the customisations you need. If that doesn't work, try adding !important after value.
e.g. 1
p.popover-text {
color: orange;
}
e.g. 2
p.popover-text {
color: orange!important;
}
You should be able to customise the expanding option as well, but I didn't poke in to that one - it used to be part of the standard stylesheet styles and is probably something like span.expandtext