Lighten dark base16 gutter slightly

src/base16/base16-dark_template.scss

 $ui-cursor-normal-mode   : $ui-cursor;
 $ui-margin-line          : transparentize($ui-selection, 0.25);
 $ui-gutter-color         : transparentize($ui-foreground, 0.5);
-$ui-gutter-background    : $ui-background;
+$ui-gutter-background    : lighten($ui-background, 2%);
 
 // less common UI colors
 $ui-line-active           : transparentize(darken($ui-background, 5%), 0.5);