trailing-whitespace missing background

[oscarfv]

trailing-whitespace only specifies foreground (which is useless for whitespace), not background, so default background is used and whitespace is invisible.

I suggest something like this:

-    `(trailing-whitespace ((,class (:foreground ,nord3))))
+    `(trailing-whitespace ((,class (:background ,nord15))))

Magenta is strong enough to act as a "warn" and is not confused with the red used for removed lines in diffs.