title: openpyxl font conditional formatting
body: <p>I am using pyopenxl to output some excel spreadsheets, I encountered a problem with font conditional formatting. I want to highlight the cells lesser than 0 with red color and here's what I've done:</p>
  
  <pre><code>from pyopenxl import formatting, styles
  
  red_font = styles.Font(size=self.font_size, bold=bold, color=self.red_color_font)
  red_fill = styles.PatternFill(start_color=self.red_color, end_color=self.red_color, fill_type='solid')
  
  self.ws.conditional_formatting.add(
      cell.coordinate,
      formatting.CellIsRule(operator='lessThan', formula=['0'], fill=red_fill, font=red_font)
  )
  </code></pre>
  
  <p>So I simply created styles for font and fill and applied them for my cell. The bad thing is that it doesn't work. As soon as I remove the font formatting from the <code>CellIsRule()</code> everything goes back to normal and I'm having my cell filled with red. But the thing is that I need to change the color as well, does anyone has any idea what's wrong with my code? Or maybe with openpyxl?</p>