Improve appearance of theme_classic

Facet strips were filled and legend keys were grey while all the rest is
black and white and as minimal as possible. Changes are made to match that
feel.

Author: jiho

R/theme-defaults.r

@@ -178,6 +178,8 @@ theme_classic <- function(base_size = 12, base_family = ""){
       panel.border     = element_blank(),
       axis.line        = element_line(colour = "black"),
       panel.grid.major = element_blank(),
-      panel.grid.minor = element_blank()
+      panel.grid.minor = element_blank(),
+      strip.background = element_rect(colour = "black", size = 0.5),
+      legend.key       = element_blank()
     )
 }