title: Removing background color for column labels while keeping plot background color ggpairs
body: <p>I have defined a function to set the background in ggpairs to match the level of correlation between two variables. 
  However, I would also like to remove the gray background from the variable labels running along the outside of the plot, but I'm not able to do this without also removing the correlation colors.</p>
  
  <pre><code>library(GGally)
  
  # Loads some data
  mtcars &lt;- dput(mtcars)[,1:6]
  
  # Defines function to color according to correlation
  cor_func &lt;- function(data, mapping, method, symbol, ...){
    x &lt;- eval_data_col(data, mapping$x)
    y &lt;- eval_data_col(data, mapping$y)
  
    corr &lt;- cor(x, y, method=method, use='complete.obs')
  
    colFn &lt;- colorRampPalette(c("brown1", "white", "dodgerblue"), 
                  interpolate ='spline')
    fill &lt;- colFn(100)[findInterval(corr, seq(-1, 1, length = 100))]
  
    ggally_text(
      label = paste(symbol, as.character(round(corr, 2))), 
      mapping = aes(),
      xP = 0.5, yP = 0.5,
      color = 'black',
      ...) + 
      theme_void() +
      theme(panel.background = element_rect(fill = fill))
  }
  
  # Following the suggestion by @Jonni
  pm &lt;- ggpairs(mtcars, 
            upper = list(continuous = wrap(cor_func,
                    method = 'spearman', symbol = expression('\u03C1 ='))),
            lower = list(continuous = function(data, mapping, ...) {
                    ggally_smooth_lm(data = data, mapping = mapping) +
                    theme(panel.background = element_blank())}),
            diag = list(continuous = function(data, mapping, ...) {
                    ggally_densityDiag(data = data, mapping = mapping) + 
                    theme(panel.background = element_blank())}
                  ))
  
  pm
  
  # All of these methods looses the correlation color in addition
  # to the background color of the labels
  pm + theme(strip.background = element_rect(fill = "white"))
  pm + theme(strip.background = element_rect(fill = NA))   
  pm + theme(strip.background = element_blank())
  
  # This only looses the correlation colors
  pm + theme(panel.grid.major = element_blank(), panel.grid.minor = 
                    element_blank())
  </code></pre>
  
  <p>Here is the plot with color as resulting from the first call to plot (still has the gray label background):</p>
  
  <p><a href="https://i.stack.imgur.com/MPxSR.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MPxSR.png" alt="enter image description here"></a></p>