%%
%% The LaTeX Companion, 3ed
%%
%% Example 8-4-23 on page I-627 in "Defining new tcolorbox environments and commands".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{87.0pt}

%StartShownPreambleCommands
  \usepackage[vignette]{tcolorbox}
  \newtcbox\key[1][white]{enhanced,sharp corners,on line,
    size=small,left=0pt,right=0pt,boxsep=1pt,fontupper=\footnotesize,
    colback=#1!10,colframe=#1!50!black,boxrule=3pt,underlay vignette}
%StopShownPreambleCommands

\begin{document}
  Use \key{Cntrl-X} followed by \key[black]{f} to open a file.
\end{document}