I am using EE 15.5.1 64 bit on windows pro 7 64 bit.
I met a problem I did not know exactly how to trigger it or turn it off.
I just remembered that I changed Marks setting and go back again it appeared.
It is as that: a single letter: a d e h i n o r s w, were highlight as red color and the space turns to pale yellow color background thru whole file.
Any suggestion is welcome to solve this problem.
BTW, I am using MiniZen theme.