For the rule colour, you can set the `\arrayrulecolor{}` from the `booktabs` package.

To get the `\rangle` and `\langle` coloured, one can smuggle some colour into `\__codedoc_meta`.

```
% \iffalse meta-comment
% Comment
% \fi
% \iffalse
%<*internal>
\iffalse
%</internal>
%<*readme>
Readme content
%</readme>
%<*internal>
\fi
%</internal>
%<*driver>
\documentclass{l3doc}
\usepackage{xcolor}
\pagecolor{blue!20!black}
\color{yellow!90!white}
\arrayrulecolor{orange}
\usepackage{fontspec}
\setmonofont[Color=green]{Arial}

\ExplSyntaxOn
\cs_set_protected:Npn \__codedoc_meta:n #1
  {
    \color{orange}
    \tl_set:Nn \l__codedoc_tmpa_tl {#1}
    \tl_map_inline:nn
      { { 3 } { 4 } { 7 } { 8 } { 11 } { 12 } { 13 } }
      {
        \tl_set_rescan:Nnn \l__codedoc_tmpb_tl
          { \char_set_catcode:nn { `_ } {##1} } { _ }
        \tl_replace_all:Non \l__codedoc_tmpa_tl \l__codedoc_tmpb_tl
          { \__codedoc_ensuremath_sb:n }
      }
    \exp_args:NV \__codedoc_meta_original:n \l__codedoc_tmpa_tl
  }

\cs_set_protected:Npn \__codedoc_syntax_end:
  {
        \end{minipage}
      \end{tabular}
      \arrayrulecolor{orange}
    \hbox_gset_end:
    \bool_if:NF \l__codedoc_in_function_bool
      {
        \begin{quote}
          \mode_leave_vertical:
          \box_use_drop:N \g__codedoc_syntax_box
        \end{quote}
      }
  }

\ExplSyntaxOff

\begin{document}
\DocInput{\jobname.dtx}
\end{document}
%</driver> 
% \fi
% \begin{documentation}
%   \section{test1}
%   \begin{function}{\test}
%     \begin{syntax}
%       \cs{test}\marg{test}
%     \end{syntax}
%     Hello world
%   \end{function}
%   \begin{function}{\test}
%     \begin{syntax}
%       \cs{test}\marg{test}
%     \end{syntax}
%     Hello world
%   \end{function}
%\section{test2}
%   \begin{function}{\test}
%     \begin{syntax}
%       \cs{test}\marg{test}
%     \end{syntax}
%     Hello world
%   \end{function}
% \end{documentation}
% \Finale 
```


![Screen Shot 2020-11-22 at 13.29.06.png](/image?hash=e2eb24f097accf0241bd17f90000036e3e0577743fccdb7b624e1811bef7d91a)