default pattern fill colors in 2.2.4
Created originally on Bitbucket by max22 (Max Böhm)
If no colors have been defined in Excel for a cell with a "pattern fill" then the styles.xml file does not contain fgColor and bgColor elements at all. In Excel such elements have WHITE background and BLACK foreground colors.
Openpyxl currently sets fgColor and bgColor both to a default value of BLACK if they are not defined. The default for bgColor should be WHITE instead.
The following change in fills.py, line 81 will fix that
def __init__(self, patternType=None, fgColor=Color(), bgColor=Color(rgb=WHITE),