documentclass{article}

usepackage{tcolorbox}
tcbuselibrary{theorems}
tcbuselibrary{skins}
newtcbtheorem[number within=section]{coloredtheorem}{Theorem}{colback=red!15{th}

begin{document}

begin{coloredtheorem}{Pigs}{pig}
Pigs are pink.
end{coloredtheorem}

end{document}

newtcbtheorem[number within=section]{coloredtheorem}{Theorem}{colback=red!15, sharp corners}{th}

Package pgfkeys Error: I do not know the key '/tcb/sharp corners' . . .

documentclass{article}

usepackage{tcolorbox}
tcbuselibrary{theorems}
tcbuselibrary{skins}
newtcbtheorem[number within=section]{coloredtheorem}{Theorem}{colback=red!15,arc=0pt,outer arc=0pt}{th}

begin{document}

begin{coloredtheorem}{Pigs}{pig}
Pigs are pink.
end{coloredtheorem}

end{document}