BUG: Limited available colors
(GH37967)

- [x] closes #37967
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Not sure if bug is the good label... Two small scripts used for that.
```
from pandas.io.formats.excel import CSSToExcelConverter
from matplotlib.colors import CSS4_COLORS

# check if existing colors match the ones of matplotlib
for name, color in CSSToExcelConverter.NAMED_COLORS.items():
    assert "#" + color == CSS4_COLORS[name]

# generate code for CSSToExcelConverter
for name, color in CSS4_COLORS.items():
    print(f'"{name}": "{color[1:]}",')
``` cc @charlesdong1991  (removing 1.2 milestone, since this is not critical to include) thanks @ma3da 