long tables: customization of rule width/thickness

[DAbidou]

For long tables, it would be great if I could make the width/thickness of the rules encompassing the first header different from those of the repeated headers. Also, the same is desired for the repeated footers and last footer.

[lvjr]

You can create your special tabularray environment and add extra rules to first head and last foot:

\documentclass{article}

\usepackage[paperwidth=8cm,paperheight=6cm,width=7cm,height=5cm]{geometry}
\usepackage{tabularray,xcolor}
\UseTblrLibrary{booktabs}

\begin{document}

\DefTblrTemplate{firsthead}{mytemplate}{
  \UseTblrTemplate{caption}{default}
  \vskip 6pt
  \hrule height 1pt
}

\DefTblrTemplate{middlehead,lasthead}{mytemplate}{
  \UseTblrTemplate{capcont}{default}
  \vskip 6pt
}

\DefTblrTemplate{firstfoot,middlefoot}{mytemplate}{
  \vskip 6pt
  \UseTblrTemplate{contfoot}{default}
}

\DefTblrTemplate{lastfoot}{mytemplate}{
  \hrule height 1pt width \tablewidth
  \vskip 6pt
  \UseTblrTemplate{note}{default}
  \UseTblrTemplate{remark}{default}
}

\NewTblrTheme{mytheme}{
  \SetTblrTemplate{head,foot}{mytemplate}
}

\NewTblrEnviron{mytblr}
\SetTblrOuter[mytblr]{long,headsep=0pt,footsep=0pt,theme=mytheme}

\begin{mytblr}[
  caption = {The Caption},
]{
  colspec = {XX}, rowhead = 1,
  row{even} = {gray!30}, row{1} = {blue!15},
}
\toprule[1.5pt]
  One & Two  \\
\midrule
   1  &   2  \\
   3  &   4  \\
   5  &   6  \\
   7  &   8  \\
   9  &  10  \\
  11  &  12  \\
  13  &  14  \\
  15  &  16  \\
  17  &  18  \\
  19  &  20  \\
\bottomrule[1.5pt]
\end{mytblr}

\end{document}