\usepackage[most]{tcolorbox}
\usepackage{graphicx}
\tcbuselibrary{breakable}

% =========================================================
% Concept Boxes
% =========================================================
\newtcolorbox{proposedbox}{
  title=Proposed Concept,
  colback=blue!5!white,
  colframe=blue!75!black,
  colbacktitle=blue!15!white,
  fonttitle=\bfseries,
  coltitle=black,
  enhanced,
  sharp corners,
  breakable,
  after = \par\vspace{6pt}
}

\newtcolorbox{establishedbox}{
  title=Established Concept,
  colback=green!5!white,
  colframe=green!75!black,
  colbacktitle=green!15!white,
  fonttitle=\bfseries,
  coltitle=black,
  enhanced,
  sharp corners,
  breakable,
  after = \par\vspace{6pt}
}

\newtcolorbox{speculativebox}{
  title=Speculative Concept,
  colback=purple!5!white,
  colframe=purple!75!black,
  colbacktitle=purple!15!white,
  fonttitle=\bfseries,
  coltitle=black,
  enhanced,
  sharp corners,
  breakable,
  after = \par\vspace{6pt}
}