what is “theme_setup” method for?

Methods in 3rd party plugin and theme classes can be called whatever the author wants them to be called. theme_setup() doesn’t mean anything special, and doesn’t even necessarily mean the same thing in different plugins.

And since it’s not a special method to WordPress, they’re never going to be called automatically by WordPress, so the plugin would be calling them somewhere, you just haven’t found it. If I had to guess, theme_setup() is a logical name for a method that might be being hooked to after_theme_setup, but there’s no guarantee.

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