title: Setting panel.border in ggplot theme turns panel background white (wipes out the plot itself)
body: <p>I would like to make the border of my plots a certain color other than black. I notice that if I set the default theme to theme_bw() using:</p>
  
  <pre><code>theme_set(theme_bw())
  </code></pre>
  
  <p>that I can set the border to the color I want using </p>
  
  <pre><code>theme(panel.border = element_rect(color="darkred", size=0.5, linetype="solid").
  </code></pre>
  
  <p>That seems to work ok. The border of each panel, whether it's a faceted plot or a single plot, takes on the "darkred" color and the rest of the plot is the same as it was before I changed the panel.border.</p>
  
  <p>However, if I use a different default theme, say theme_gray() or theme_classic(), then the border changes but the contents of each of the facets is wiped out (completely white).</p>
  
  <p>Any idea what is causing this difference in behavior or what I can do to fix it?  I would like to use theme_gray() and put a thin colored line around the border of each facet.  </p>