Reaper looks for buttons in whatever folder the current theme is configured to use. If it doesn't find a file, it substitutes the default file.
You can change the button folder a theme uses in Prefs > Color Theme.
If you want to experiment with mixing & matching different buttons, i'd suggest you duplicate a folder for a theme you like, change your theme to point to the new folder (be sure to save the theme), then copy buttons from whatever other theme you want to use and overwrite the original buttons.
You don't HAVE to create a new folder, but that way, the original theme is still intact.
tj
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