\documentclass{article}
\usepackage{acro}

\DeclareAcroArticle{definite}{the}
\DeclareAcroEnding{possessive}{'s}{'s}

\NewAcroCommand\dac{m}{\acrodefinite\UseAcroTemplate{first}{#1}}
\NewAcroCommand\Dac{m}{\acroupper\acrodefinite\UseAcroTemplate{first}{#1}}

\NewAcroCommand\Dacg{m}{%
  \acroupper
  \acrodefinite
  \acropossessive
  \UseAcroTemplate{first}{#1}%
}

\DeclareAcronym{css}{
  short = CSS ,
  long = Centre for Spaghetti Studies ,
  short-definite = \nospace ,
  long-possessive = '
}

\begin{document}

Bob hails from \dac{css}. \par
Bob hails from \dac{css}.

\acreset{css}

\Dacg{css} mandate is broad. \par
\Dacg{css} mandate is broad.

\acreset{css}

\Dac{css} scientists eat well. \par
\Dac{css} scientists eat well. \par

\end{document}
