一般来说：theme()对文本标签没有影响。它设置图的整体外观，只影响所谓的非数据墨迹。对于文本标签，您必须在geom或stat中设置字体。但是，stat_cor使用?plotmath表达式来创建标签，也就是说，使R或.斜体。并且在标签的表面不能通过fontface=设置或更改。
因此，要使标签加粗，需要在after_stat中自己创建标签：

library(ggplot2)
library(ggpubr)

ggplot(iris, aes(x = Sepal.Width, y = Sepal.Length)) +
  geom_point(aes(color = Species)) +
  stat_smooth(method = lm, aes(color = Species), se = FALSE) +
  stat_cor(
    aes(
      color = Species,
      label = after_stat(
        paste0(
          "bolditalic(R)~",
          "bold(`=`)~",
          "bold('", r, "')"
        )
      )
    ),
    r.accuracy = 0.01,
    label.x = 4, label.y = c(8, 7.8, 7.6),
  ) +
  scale_color_manual(values = c("grey70", "gold", "green3")) +
  theme_classic() +
  theme(
    legend.background = element_rect(
      fill = "grey90",
      size = 0.5, linetype = "solid"
    )
  )
#> Warning: The `size` argument of `element_rect()` is deprecated as of ggplot2 3.4.0.
#> ℹ Please use the `linewidth` argument instead.
#> This warning is displayed once every 8 hours.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.
#> `geom_smooth()` using formula = 'y ~ x'

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