Beamer comes installed with many themes and color themes. But, there are many custom themes out there you might be interested to use. A good list of such custom themes is maintained by Rogier Koppejan here.
To install a custom Beamer theme:
.sty files of the theme to the respective subdirectories under the
$(MIKTEX_ROOT)\tex\latex\beamer\base\themes\ directory. For example,
beamerthemeTorino.sty goes into the
beamercolorthemechameleon.sty goes into the
color subdirectory and so on.
Refresh the MikTeX file name database. You may need to close all applications that use LaTeX for this operation to complete successfully.