Ink and paper settings for built-in themes (re: #6059) (#6063)

* abstract away theme colours

* add snapshot test

* add news bullet

Author: teunbrand

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* Built-in `theme_*()` functions now have `ink` and `paper` arguments to control
+  foreground and background colours respectively (@teunbrand)
 * The `summary()` method for ggplots is now more terse about facets 
   (@teunbrand, #5989).
 * `guide_bins()`, `guide_colourbar()` and `guide_coloursteps()` gain an `angle`