Hi, @yuna-icedcoffee! You don't have to open or unzip the file. Just download from the given link and use the "Import Profile" button from the Chess.com Custom Pieces extension. After clicking "Import Profile", select the file that you just downloaded(it will be a .cext file) and the theme will be applied on your chess.com board and pieces.
Valentines Day Chess Piece Set/Theme
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That's strange, it works fine for me. Alternatively, you can download the individual PGN files from here and then try uploading them.
Valentine's Day Piece Set/Theme
You can download the piece set/theme from here and use the Import Profile button using the Chess.com Custom Pieces extension to set the theme. Enjoy!