Okay. I think I found it. I decided to do another search for "Moodle theme larrow", and came across this - https://moodle.org/mod/forum/discuss.php?d=252153
I looked at the custom theme, and it was setting a value for $THEME->rarrow in its config.php file, but not one for $THEME->larrow. I added the line:
$THEME->larrow = " ";
and the problem went away.