library(ggplot2)

ID <- 1:600
values <- c(rnorm(150, mean=25, sd=5), rnorm(150, mean=30, sd=4), rnorm(150, mean=15, 3), rnorm(150, mean=40, sd=5))
category <- c(rep("blue", 150), rep("red", 150), rep("green", 150), rep("yellow", 150))
df <- data.frame(ID, category, values)
baseplot <- ggplot(df, aes(category, values, fill=category)) +
  geom_boxplot() +
  scale_fill_manual(values=c("blue", "green", "red", "yellow"))
baseplot

#remove theme
baseplot +
  theme_void()

#predefined theme example1
baseplot +
  theme_dark()

#predefined theme example2
baseplot +
  theme_classic()

#changing the background color of the data area of the plot
baseplot +
  theme(panel.background = element_rect(fill = "lightblue"))

#changing the background color of the surrounding area of the plot
baseplot +
  theme(plot.background = element_rect(fill = "lightblue"))

#changing the background color of the surrounding area of the plot
baseplot +
  theme(plot.background = element_rect(fill = "lightblue"))

#changing the grid
baseplot +
  theme(
    panel.grid.major = element_line(colour="darkblue", size=0.5, linetype="solid"),
    panel.grid.minor = element_line(colour="lightblue", size=0.4, linetype="dashed"))

#removing the grid
baseplot +
  theme(
    panel.grid.major = element_blank(),
    panel.grid.minor = element_blank())