Setting Theme in manifest is usually used for Activity.
If you want to set Theme for Fragment, add next code in the onGetLayoutInflater() of the Fragment:
override fun onGetLayoutInflater(savedInstanceState: Bundle?): LayoutInflater {
val inflater = super.onGetLayoutInflater(savedInstanceState)
val contextThemeWrapper: Context = ContextThemeWrapper(requireContext(), R.style.yourCustomTheme)
return inflater.cloneInContext(contextThemeWrapper)
}