Theme palettes warn about options

[teunbrand]

This PR aims to fix #6333.

Briefly, theme() checks whether colour palettes have been set simultaneously in the options and emit a warning when they are.

RE: my earlier comment:

Mechanically, would you propose to detect this situation whenever theme() is called, or when scale_colour_*() is called?

I think theme() is the more appropriate place because:

• theme() has access to the theme palette and the options
• scale_colour_*() never gets access to the theme, and the alternative would be ScalesList$set_palettes(), at which point it would be too late the change the entire scale rather than just the palette.
• I imagine options() is more used in a global fashion than theme(). The case that we'd be missing now is if we do a theme_set() with a palette before options().
• An alternative is to do this in check_theme(), which runs every render.

Using theme without options is silent:

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

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

p + theme(palette.colour.discrete = c("dodgerblue", "orange", "orchid"))

Using options without theme settings is silent:

options(ggplot2.discrete.colour = c("red", "yellow", "limegreen"))
p

But using both emits a warning.

p + theme(palette.colour.discrete = c("dodgerblue", "orange", "orchid"))
#> Warning: The `options('ggplot2.discrete.colour')` setting is incompatible with the
#> `palette.colour.discrete` theme setting.
#> ℹ You can set `options(ggplot2.discrete.colour = NULL)`.

^{Created on 2025-02-17 with reprex v2.1.1}

[teunbrand]

I've separated this in its own function as all these deprecations became somewhat distracting in ascertaining what theme() actually does.

[thomasp85]

LGTM