Make legend.box.background work for theme_void() (tidyverse#2725)

* Provide default option in if--else chain. Closes tidyverse#2723.

* add legend.box setting to theme_void()

* use `identical` instead of `==`.

Author: clauswilke

R/guides-.r

@@ -248,7 +248,7 @@ guides_build <- function(ggrobs, theme) {
   yjust <- just[2]
 
   # setting that is different for vertical and horizontal guide-boxes.
-  if (theme$legend.box == "horizontal") {
+  if (identical(theme$legend.box, "horizontal")) {
     # Set justification for each legend
     for (i in seq_along(ggrobs)) {
       ggrobs[[i]] <- editGrob(ggrobs[[i]],
@@ -263,7 +263,7 @@ guides_build <- function(ggrobs, theme) {
     # add space between the guide-boxes
     guides <- gtable_add_col_space(guides, theme$legend.spacing.x)
 
-  } else if (theme$legend.box == "vertical") {
+  } else { # theme$legend.box == "vertical"
     # Set justification for each legend
     for (i in seq_along(ggrobs)) {
       ggrobs[[i]] <- editGrob(ggrobs[[i]],

R/theme-defaults.r

@@ -430,6 +430,7 @@ theme_void <- function(base_size = 11, base_family = "",
     axis.text =          element_blank(),
     axis.title =         element_blank(),
     axis.ticks.length =  unit(0, "pt"),
+    legend.box =         NULL,
     legend.key.size =    unit(1.2, "lines"),
     legend.position =    "right",
     legend.text =        element_text(size = rel(0.8)),