title: Change ggplot2 colourbar tick marks to black
body: <p>In some of my plots I find it hard to see the tick marks in the colour bar. I haven't been able to find a documented way to change the colour of the ticks. All the examples seem to focus on changing the labels or not drawing ticks at all. Is it possible?</p>
  
  <pre><code>#  Data
  require(ggplot2)
  require(grid)
  n &lt;- 100
  x &lt;- y &lt;- seq(-4*pi, 4*pi, len=n)
  r &lt;- cos( sqrt( outer(x^2, y^2, "+") ) ^ 2 )
  df &lt;- data.frame( x = rep( x , each = n) , y = rep( y , times = n ) , val = c(r) )
  
  #  Plot
  ggplot( df , aes( x , y , fill = val ) )+
    geom_raster()+
    scale_fill_gradient( low = "#FFFFFF" , high = "#de2d26" )+
    guides( fill = guide_colourbar( barheight = unit( 3 , "in" ) ) )+
    theme_bw()+
    theme( line = element_line( colour = "#0000FF" ) )
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/ML7Uh.jpg" alt="enter image description here"></p>
  
  <p><strong>How can I make the ticks in the colourbar be plotted in black rather than white, without changing other elements of the plot?</strong></p>
  
  <hr>
  
  <p>p.s. kudos to <a href="https://stackoverflow.com/q/19356350/1478381"><strong>this question</strong></a> for the function to create the example data</p>