conf.py

latex_elements = {
    'extrapackages': r'\usepackage{titlesec}',
    'preamble': '\n'.join([
        # 字下げ (なんでデフォルトで動かないんだろ?)
        r'\setlength{\parindent}{1zw}',
        # 段落調整、箇条書きの行間調整
        r'\setlength{\parskip}{0ex}',
        # 見出しの装飾
        # ref: https://qiita.com/krtx/items/deeec0843caf37b66054
        r'\definecolor{teal}{RGB}{0,128,128}',
        r'\definecolor{powderblue}{RGB}{176,224,230}',
        r'\definecolor{darkslateblue}{RGB}{72,61,139}',
        r'\definecolor{darkslategray}{RGB}{47,79,79}',
        r'\definecolor{lightcyan}{RGB}{224,255,255}',
        r'',
        r'% section',
        r'\titleformat{\section}[block]',
        r'{}{}{0pt}',
        r'{',
        r'  \colorbox{teal}{\begin{picture}(0,20)\end{picture}}',
        r'  \hspace{0pt}',
        r'  \normalfont \Huge\bfseries \thesection',
        r'  \hspace{-4pt}',
        r'}',
        r'[',
        r'\begin{picture}(100,0)',
        r'  \put(3,30){\color{teal}\line(1,0){400}}',
        r'\end{picture}',
        r'\\',
        r'\vspace{-50pt}',
        r']',
        r'',
        r'% subsection',
        r'\titleformat{\subsection}[block]',
        r'{}{}{0pt}',
        r'{',
        r'  \colorbox{darkslateblue}{\begin{picture}(0,10)\end{picture}}',
        r'  \hspace{0pt}',
        r'  \normalfont \Large\bfseries \thesubsection',
        r'  \hspace{-4pt}',
        r'}',
        r'[',
        r'\begin{picture}(100,0)',
        r'  \put(3,18){\color{darkslateblue}\line(1,0){300}}',
        r'\end{picture}',
        r'\\',
        r'\vspace{-30pt}',
        r']',
    ])

ありがとう〜〜 参照リンクはらせてもらいました
https://scrapbox.io/shimizukawa/SphinxのLaTeX_PDF出力をカスタマイズする