BUG: latex css conversion does not work on new applymap_index

[attack68]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the master branch of pandas.

Reproducible Example

df = DataFrame([[0], [1]], index=[0, 1])
styler = df.style
styler.applymap(lambda x: "color: red;" if x == 0 else "")
styler.applymap_index(lambda x: "color: blue;" if x == 0 else "")
print(styler.to_latex(convert_css=True))

\begin{tabular}{lr}
 & 0 \\
\colorblue 0 & \color{red} 0 \\
1 & 1 \\
\end{tabular}

Issue Description

the new method applymap_index has not been hooked up to the CSS conversion for styler.to_latex.

Expected Behavior

should convert to color{blue}

Installed Versions

master