title: openpyxl conditional formatting - Rule is in Excel file but not the formatting
body: <p>My goal is to create an Excel file and change the background color of some cells based on their value using conditional formatting, using openpyxl.</p>
  
  <p>When I open the file created with Excel, I can see that the rule is there, but the rule does not include the formatting to apply (background color set to none). The cells have thus no background color, although the border of the cells respecting the formula are not visible, like when the background is white.
  I don't see if I made a mistake, or if there is some trouble with openpyxl.</p>
  
  <p>Here is a MWE:</p>
  
  <pre><code>from openpyxl import Workbook
  from openpyxl.styles import PatternFill
  from openpyxl.formatting.rule import CellIsRule
  
  wb = Workbook()
  ws = wb.active
  
  ws['B2'] = -2
  ws['B3'] = -1
  ws['B4'] =  0
  ws['C2'] = -1
  ws['C3'] =  0
  ws['C4'] =  1
  
  fill = PatternFill(start_color='538DD5', fill_type='solid')
  ws.conditional_formatting.add('B2:C4', CellIsRule(operator='lessThan', formula=[0], fill=fill))
  
  wb.save('mwe.xlsx')
  wb.close()
  </code></pre>