I am going to provide my code below. there is my plot and at the sample plot i need to recreate. I don’t know how to add exactly same grey thick borders.

Current plot

Desired

ggplot(data = melted_df, aes(sample = value, color = Distribution)) +
  stat_qq() +
  geom_qq_line() +
  facet_wrap(~Distribution, scales = "free") + xlab('Theoretical Quantiles') + 
  ylab('Sample Quantiles') + 
  ggtitle('QQ Plot for Different Distributions against Qnorm!') + 
  theme(panel.grid.major = element_blank(), panel.grid.minor = element_blank(), plot.title = element_text(size = 11),
        panel.background = element_rect(fill = "white"), legend.background=element_rect(fill = "lightgray"),
        legend.key = element_rect(fill = "lightgray"),
        axis.ticks.x = element_blank(),
        axis.ticks.y = element_blank(),
        legend.box.background = element_rect(color = "gray45", fill = NA, size=3),
        panel.border = element_rect(color = "gray45",fill = NA, size = 2), 
        strip.text = element_text(size = 10))

>Solution :

You are looking for strip.background. As the docs state:

strip.background background of facet labels (element_rect(); inherits from rect).

In your case to match your other borders you can set it to element_rect(color = "gray45",fill = "grey90", size = 2)

Here’s an example with the iris dataset:

ggplot(iris, aes(sample = Sepal.Length, color = Species)) +
     stat_qq() +
  geom_qq_line() + 
  facet_wrap(~Species, scales = "free") + xlab('Theoretical Quantiles') + 
  ylab('Sample Quantiles') + 
  ggtitle('QQ Plot for Different Distributions against Qnorm!') + 
  theme(panel.grid.major = element_blank(), panel.grid.minor = element_blank(), plot.title = element_text(size = 11),
        panel.background = element_rect(fill = "white"), legend.background=element_rect(fill = "lightgray"),
        legend.key = element_rect(fill = "lightgray"),
        axis.ticks.x = element_blank(),
        axis.ticks.y = element_blank(),
        legend.box.background = element_rect(color = "gray45", fill = NA, size=3),
        panel.border = element_rect(color = "gray45",fill = NA, size = 2), 
        strip.text = element_text(size = 10),
        legend.position  = "bottom", # I prefer the legend at the bottom
        strip.background = element_rect(color = "gray45",fill = "grey90", size = 2)
        # ^^ the line above is the only thing you need to add
    )