SWI-Prolog -- prolog:theme

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[hook]prolog:theme_hook(+Theme)

Multifile hook that is called if the Prolog flag theme is about to get the new value Theme. The hook is called as forall(prolog:theme_hook(Theme), true) in the thread that sets the flag, before the flag is changed. If the hook raises an exception, the flag keeps its old value and set_prolog_flag/2 raises the exception. The system defines a clause that loads library(theme/Theme) if this file exists and Theme is not yet defined, i.e., prolog:theme(Theme) is false.