Since a Label in a PopupMenuItem, I thinks it can be set a pango text here.
PopupMenuItem: A PopupBaseMenuItem that displays text in a St.Label.
const item = new PopupMenu.PopupMenuItem("");
item.actor.set_size(300,150); <--- effect.
item.actor.create_pango_layout('<span foreground="blue" size="32">fname</span>'); <- not effect
item.actor.set_markup('<span foreground="blue" size="32">fname</span>'); <-- fail, but it worked in vala.
I read popupMenu.js source code before, but now I forgot the url.
You probably want to set this on the internal Clutter.Text of the PopupMenu.PopupMenuItems's St.Label:
const item = new PopupMenu.PopupMenuItem('');
item.label.clutter_text.set_markup(
'<span foreground="blue" size="32">fname</span>');
You can bookmark this link to the whole directory of GNOME Shell's UI: