\catcode`\@=11
\let\latex@begintheorem=\@begintheorem
\let\latex@opargbegintheorem=\@opargbegintheorem
\def\my@begintheorem#1#2{\trivlist
   \item[\hskip \labelsep{{\bfseries #1\ #2}:}]\itshape}
\def\my@opargbegintheorem#1#2#3{\trivlist
      \item[\hskip \labelsep{{\bfseries #1\ #2}:\ {\bfseries #3}:}]\itshape}
\renewcommand{\@begintheorem}{\my@begintheorem}
\renewcommand{\@opargbegintheorem}{\my@opargbegintheorem}
\catcode`\@=12

