Why does the gutter show up in a different color?
Why is the left edge of my window (also known as "the gutter") a different color?
You are either using a different "Line Prefix Area" color, or you have modified lines turned on.
To change the color of your "Line Prefix Area", go to Tools > Options > Appearance > Colors and set the "Line Prefix Area" color to match your "Window Text" color.
To toggle the "Modified Lines" feature, go to the Tools > Options > Languages > [select language] > View and toggle the "Modified Lines" checkbox.