If your tag is meant for ggplot2 answers.
# data preparation
df = data.frame(Group = LETTERS[1:9],
Yellow = c(-0.61,0.13,0.24,-0.30,-0.12,-1.24,0.74,0.55,0.80),
Blue = c(1.02,-0.40,0.73,0.17,0.68,1.21,-1.35,-0.84,-1.27)
)
library(reshape2)
df.m <- melt(df,
id.vars= c("Group"),
measure.vars= c("Yellow", "Blue"),
variable.name= "Color",
value.name= "val"
)
# plot
library(ggplot2)
ggplot(data=df.m, aes(x=Group, y=val, group= Color, colour=Color, fill=Color)) +
geom_point(size=2) +
geom_polygon(size = 1, alpha= 0.2) +
ylim(-2.0, 2.0) + ggtitle("Radar") +
scale_x_discrete() +
theme_light()+
scale_color_manual(values= c("yellow", "blue"))+
scale_fill_manual(values= c("yellow", "blue"))+
coord_polar()

In addition to its theme, some aesthetic parameters such as point size (geom_point(size=2)), line thickness (geom_polygon(size = 1)), color (scale_color_manual()) and filling color (scale_fill_manual()) could be useful to modify its look.