run in a terminal: kpsewhich beamerthemedefault.sty. and you'll get the exact position of that file. However, that is the official directory, where ...
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This package includes 201 files with a total size of 4892546 bytes. Name, Directory. beamer.cls, texmf\tex\latex\beamer. beamerarticle.sty, texmf\tex .... This theme matrix shows examples of all variations of the Beamer LaTeX package's default themes and color themes. 3925e8d270
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