%%
%% The LaTeX Companion, 3ed
%%
%% Example 8-4-7 on page I-619 in "Skins and styles --- altering the look and feel".
%%
%% 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{105.0pt}

\IfPackageLoadedTF{colorspace}
                  {\AtBeginDocument{\footnotesize \tcbset{colframe=spotblue!75!spotblack}}}
                  {\AtBeginDocument{\footnotesize \tcbset{colframe=blue!75!black}}}

%StartShownPreambleCommands
\usepackage{tcolorbox}   \tcbuselibrary{skins}
\tcbset{skin=enhanced,colframe=blue!75!black,fonttitle=\bfseries,
        interior style={left color=red!20,right color=white}}
%StopShownPreambleCommands

\begin{document}
\begin{tcolorbox}[halign lower=center,title=Interior style change]
  Here the background \tcblower color changes from red (gray) to white.
\end{tcolorbox}
\end{document}