just use font=2
as argument.
plot.xts(Margins,
screens = c(1,1,2,2,3,3,4,4),
col = brewer.pal(n=9,"PuBu")[c(8,4,6,9)], lwd = 6,
legend.loc = c("topright"), auto.legend = T,
main="Margin Comparison",
font = 2
)