Ink and paper settings for built-in themes

[teunbrand]

This PR aims to fix an issue discussed during conf.

Briefly, treading in the footsteps of #5833, we give ink and paper argument to built-in themes.

The ink sets all foreground elements, which by default is 'black'. Likewise, paper sets all background elements, which by default is 'white'.

Some examples of non-default options:

Changing the foreground

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2
p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point()

p + theme_bw(ink = "dodgerblue")

Changing the background

p + theme_gray(paper = "orchid")

Cheap dark-mode option

p + theme_light(ink = "white", paper = "black")

^{Created on 2024-08-26 with reprex v2.1.1}

[teunbrand]

Closing this in favour of #6063