title: Control color of legend elements that are not colour guides in ggplot
body: <p>When using a ggplot2 theme with a black background, can one control the legend colour for guides other than the colour guide so that things are not plotted in black?  If so, how?</p>
  
  <pre><code>library(ggplot2) # needs to be 0.9.3 for this theme
  data(iris)       # included with ggplot2
  
  theme_black&lt;- function (base_size = 16, base_family = ""){
      theme_minimal() %+replace% 
          theme(
                line = element_line(colour = "white", size = 0.5, linetype = 1, 
                          lineend = "butt"), 
                rect = element_rect(fill = "white", 
                          colour = "white", size = 0.5, linetype = 1), 
                text = element_text(family = base_family, 
                          face = "plain", colour = "white", size = base_size,
                          angle = 0, lineheight = 0.9, hjust = 0, vjust = 0),
                plot.background = element_rect(colour = 'black', fill = 'black'),
                plot.title = element_text(size = rel(1.2)),
                panel.border = element_rect(fill = NA, colour = "white"), 
                panel.grid.major = element_line(colour = "grey20", size = 0.2), 
                panel.grid.minor = element_line(colour = "grey5", size = 0.5),
                strip.background = element_rect(fill = "grey30", colour = "grey30")
               )
      }
  
  ggplot(data=iris, aes(x=Sepal.Length, y=Sepal.Width, shape=Species,
         colour=Petal.Length))+geom_point()+theme_black()+
         scale_colour_gradient(low = "purple", high = "white")
  </code></pre>
  
  <p>As you can see, the default colour for the shape part of the legend has not been changed, so it is invisible and one cannot tell which species is which:</p>
  
  <p><img src="https://i.stack.imgur.com/F1dB8.png" alt="enter image description here"></p>
  
  <p>The only solution I have right now is to change the legend.background colour, but this is a waste of ink and ugly. </p>