preserve theme class (#5823)

Author: teunbrand

R/theme.R

@@ -576,8 +576,9 @@ plot_theme <- function(x, default = theme_get()) {
   # Check that all elements have the correct class (element_text, unit, etc)
   validate_theme(theme)
 
-
-  theme[intersect(names(theme), names(get_element_tree()))]
+  # Remove elements that are not registered
+  theme[setdiff(names(theme), names(get_element_tree()))] <- NULL
+  theme
 }
 
 #' Modify properties of an element in a theme object