axis=None in .background_gradient is a newer feature. If it doesn't work, upgrade pandas by pip3 install --upgrade pandas.
import pandas as pd
df = # pd.DataFrame
df.style \
.background_gradient('coolwarm',axis=None)
# axis=None: colormap is defined using the values in the entire output
# axis=0: columnwise, axis=1: rowwise
.set_precision(2) \
.set_properties(**{'font-size':'0pt'})
