.bcstm file: how to open it on Windows

1 Windows app registers the .bcstm extension. Install one of them to open the file with a double-click.

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  3DS Theme Editor

  Custom creator for Nintendo 3DS Home Menu themes. Once created, they can optionally be copied over to a softmodded (or potentially emulated) 3DS/2DS.

  Open source 1.0.12.1 ★ 246

Related guide: Change the default app for a file type in Windows