Background for theme_void() and theme_minimal()

[teunbrand]

This PR aims to fix #6251.

Briefly, some themes had plot.background = element_blank() (explicitly or implicitly), which led to counterintuitive colouring.
By providing this element we surprise less.

Examples:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point()

p + theme_minimal(paper = "cornsilk")

p + theme_void(paper = "cornsilk")

^{Created on 2025-02-24 with reprex v2.1.1}

[thomasp85]

Curious why you are not just setting plot.background to a non-blank element?

[teunbrand]

You're right to wonder about this! Some test was protesting that we couldn't inherit from a blank root element. I then tried plot.background = element_rect(..., inherit.blank = FALSE) , but that gets overruled by theme(..., complete = TRUE). I then tried using +theme(...) at the end of the function to force inherit.blank = FALSE without being bound to complete = TRUE, but this felt like a dirty solution. I've landed on the current solution because at least it is straightforward.

[thomasp85]

that makes sense (in its own little way)

[thomasp85]

LGTM