使用pandas数据帧，在以HTML格式输出到电子邮件正文时，如何为多索引的两个级别设置背景色？

import pandas as pd
from IPython.display import display, HTML

data = {'A': [1, 2, 3, 4, 5],
        'B': [6, 7, 8, 9, 10],
        'C': [11, 12, 13, 14, 15]}
index = pd.MultiIndex.from_tuples([('Group 1', 'Subgroup 1'),
                                   ('Group 1', 'Subgroup 2'),
                                   ('Group 2', 'Subgroup 1'),
                                   ('Group 2', 'Subgroup 2'),
                                   ('Group 3', 'Subgroup 1')],
                                  names=['Group', 'Subgroup'])
df = pd.DataFrame(data, index=index)

styler = df.style
styler.background_gradient(subset=[('A', 'Subgroup 1'), ('B', 'Subgroup 2')], cmap='Blues')

html = styler.to_html()
display(HTML(html))

import pandas as pd
from IPython.display import display, HTML

data = {'A': [1, 2, 3, 4, 5],
        'B': [6, 7, 8, 9, 10],
        'C': [11, 12, 13, 14, 15]}
index = pd.MultiIndex.from_tuples([('Group 1', 'Subgroup 1'),
                                   ('Group 1', 'Subgroup 2'),
                                   ('Group 2', 'Subgroup 1'),
                                   ('Group 2', 'Subgroup 2'),
                                   ('Group 3', 'Subgroup 1')],
                                  names=['Group', 'Subgroup'])
df = pd.DataFrame(data, index=index)

styler = df.style
styler.background_gradient(subset=[('A', 'Subgroup 1'), ('B', 'Subgroup 2')], cmap='Blues')

html = styler.to_html()
display(HTML(html))