从 ggplot2 map 中删除白色背景

Question

我为一个项目创建了下面的map，我选择了map的背景为黑色，但是图像的左右总是有这个白色背景。 这是我用来生成它的代码：

ggplot(merged) +
geom_sf(aes(fill = value/10000000000), color = NA, alpha = 0.9) +
geom_sf(data = borders, fill = NA, size = 0.1, col = "grey30") +
coord_sf(
  xlim = c(2377294, 6500000),
  ylim = c(1413597, 5228510)
) + scale_fill_gradient2(
  name = "Euros (Billions)",
  guide = guide_legend(
  direction = "horizontal",
  keyheight = 0.5,
  keywidth = 2,
  title.position = "top",
  title.hjust = 0,
  label.hjust = .5,
  nrow = 1,
  byrow = TRUE,
  reverse = FALSE,
  label.position = "bottom"
)
) + theme_void()+
labs(
title = "Trade Balance of EU-28 Countries (2020)",
subtitle = "(Billions of Euros)",
caption = paste0("Source: Eurostat")) +
# Theme
theme(
plot.background = element_rect(fill = "black"),
plot.title = element_text(
  color = "grey90",
  hjust = 0.5,
  vjust = -1,
),
plot.subtitle = element_text(
  color = "grey90",
  hjust = 0.5,
  vjust = -2,
  face = "bold"
),
plot.caption = element_text(
  color = "grey90",
  size = 6,
  hjust = 0.5,
  margin = margin(b = 2, t = 13)
),
legend.text = element_text(
  size = 7,
  color = "grey90"
),
legend.title = element_text(
  size = 7,
  color = "grey90"
),
legend.position = c(0.5, 0.02),
)

如果有人可以就如何去除侧面的白色提供一些帮助，我将不胜感激。 谢谢。