tcolorbox — Colored & Framed Boxes

CTAN: https://ctan.org/pkg/tcolorbox
Manual: texdoc tcolorbox (~500 pages)

Setup

\usepackage[most]{tcolorbox}
% 'most' loads: skins, breakable, listings, theorems, fitting, hooks, xparse
% For minimal: \usepackage{tcolorbox} then \tcbuselibrary{...}

\tcbuselibrary{skins}       % enhanced appearance
\tcbuselibrary{breakable}   % boxes across pages
\tcbuselibrary{theorems}    % theorem integration
\tcbuselibrary{listings}    % code listings in boxes
\tcbuselibrary{listingsutf8} % UTF-8 code listings