Вы можете использовать wrap
для содержимого требуемого тега.
import pandas
from bs4 import BeautifulSoup
a = [['1/2/2014', 'a', '6', 'z1'],
['1/2/2014', 'a', '3', 'z1'],
['1/3/2014', 'c', '1', 'x3']]
df = pandas.DataFrame.from_records(a[1:], columns=a[0])
soup = BeautifulSoup(df.to_html(header=False), 'lxml')
original_tag = soup.find_all('td')[4]
new_tag = soup.new_tag('FONT', COLOR='white')
<strong>original_tag.contents[0].wrap(new_tag)</strong>
print(soup)