Use markdown in theme element after setting a complete theme

Sometimes users want to use settings from a ggplot complete theme (theme_minimal, for instance) and then manually set another theme element afterwards by calling theme().

For instance, this works

library(tidyverse)
library(ggtext)

ggplot(
  economics, aes(date, unemploy)
  ) + 
  geom_line() +
  scale_x_date(
    breaks = seq.Date(
      "1970-01-01" %>% as.Date,
      "2012-01-01" %>% as.Date, 
      length.out = 5
      ),
    labels = str_c(
      "**",
      seq(1970, 2010, by  = 10),
      "**"
      )
  ) +
  theme(
    axis.text.x =  element_markdown()
  ) 

(with a simplified example of markdown in the axis labels -- I know I could just set axis labels to be bold in theme, this is just a simplified example.)

Now trying to use theme_minimal() and then use theme(), none of the theme() settings work:

ggplot(
  economics, aes(date, unemploy)
  ) + 
  geom_line() +
  scale_x_date(
    breaks = seq.Date(
      "1970-01-01" %>% as.Date,
      "2012-01-01" %>% as.Date, 
      length.out = 5
      ),
    labels = str_c(
      "**",
      seq(1970, 2010, by  = 10),
      "**"
      )
  ) +
  theme_minimal() +
  theme(
    axis.text.x =  element_markdown()
  ) 

how do I set a theme() element after using a ggplot complete theme?

With help from @teunbrand, this is a new... design choice?... shipping with ggplot2 4.0 and their use of S7 internals.

the fix -- use axis.text.x.bottom

library(tidyverse)
library(ggtext)

ggplot(
  economics, aes(date, unemploy)
) + 
  geom_line() +
  scale_x_date(
    breaks = seq.Date(
      "1970-01-01" %>% as.Date,
      "2012-01-01" %>% as.Date, 
      length.out = 5
    ),
    labels = str_c(
      "**",
      seq(1970, 2010, by  = 10),
      "**"
    )
  ) +
  theme_minimal() +
  theme(
    axis.text.x.bottom =  element_markdown()
  )