\newtheorem{theorem}{\sf Theorem}
will cause theorem headings to be printed in sans serif type.
However, it is not easy to slip a punctuation mark after
the theorem number. This can now be done by the statement\renewcommand{\thmcounterend}{<punctuation>}
where <punctuation> is the desired punctuation. The
default is a period. This command affects all user defined
theorem environments.
The default typestyle within the theorem environment has been
changed to \sl. This can easily be overridden.