In diff mode, for cursorline, use underline styling instead of background styling to improve readability.

Fixes #107.

colors/onedark.vim

@@ -171,13 +171,17 @@ call s:h("Todo", { "fg": s:purple }) " anything that needs extra attention; most
 " }}}
 
 " Highlighting Groups (descriptions and ordering from `:h highlight-groups`) {{{
-
 call s:h("ColorColumn", { "bg": s:cursor_grey }) " used for the columns set with 'colorcolumn'
 call s:h("Conceal", {}) " placeholder characters substituted for concealed text (see 'conceallevel')
 call s:h("Cursor", { "fg": s:black, "bg": s:blue }) " the character under the cursor
 call s:h("CursorIM", {}) " like Cursor, but used when in IME mode
 call s:h("CursorColumn", { "bg": s:cursor_grey }) " the screen column that the cursor is in when 'cursorcolumn' is set
-call s:h("CursorLine", { "bg": s:cursor_grey }) " the screen line that the cursor is in when 'cursorline' is set
+if &diff
+  " Don't change the background color in diff mode
+  call s:h("CursorLine", { "gui": "underline" }) " the screen line that the cursor is in when 'cursorline' is set
+else
+  call s:h("CursorLine", { "bg": s:cursor_grey }) " the screen line that the cursor is in when 'cursorline' is set
+endif
 call s:h("Directory", { "fg": s:blue }) " directory names (and other special names in listings)
 call s:h("DiffAdd", { "bg": s:green, "fg": s:black }) " diff mode: Added line
 call s:h("DiffChange", { "bg": s:yellow, "fg": s:black }) " diff mode: Changed line