I understand that you want to change the background grey into a dark blue?
I seldom use ggplot2 so I had to google how to do that: searching for 'ggplot2 background'.
One of the first results suggested the theme option panel.background.
I inserted that in your code and removed the scale_color_gradient2 that plays no role here (?)
It possible to specify exactly the color you need by using rgb codes. See e.g. here. (Also found this page by googling).
ggplot(OEQ, aes(factor(hedonics), Percentage, fill=age)) +
theme(axis.title.x = element_blank()) +
geom_bar(stat="identity", position = "dodge", width = 1) +
# scale_color_gradient2(low = "red",high = "white") + # removed
theme(legend.title = element_blank()) + theme(legend.position = c(.80, .80)) +
theme(axis.text.x = element_text(face="bold", size=7),
axis.text.y = element_text(face="bold", size=10),
axis.title.y = element_text(size=10, face="bold"),
panel.background = element_rect(fill = "darkblue") #inserted
) +
scale_x_discrete(labels=c("5", "", "","", "Extremely\ndifficult"))