[det,multifile]prolog:line_editor_attributes(+Off,
+On)input
in effect while the user is typing (see
decorate_prompt/2 in boot/toplevel.pl).
Erasing part of the screen paints it with the current background colour,
so libedit must switch these attributes off before it erases the rows
the line no longer uses, e.g. after unwrapping a long line or moving
from a multi-line history entry to a shorter one. See EL_ATTRS in
libedit.
The toplevel calls this each time it builds a prompt, so a theme loaded at run time is picked up. Does nothing if libedit is too old to support this.