Liferay theme develop - $theme.getSetting()

in liferay-look-and-feel.xml, we can write settings like

<setting key="my-greeting" value="This is Tina's greeeeeeeeeeting!"></setting>

In portal_normal.vm, we can use $theme.getSetting("my-greeting") to get the value. I use Liferay EE 6.1.20, it doesn't work.

But if we use $themeDisplay.getTheme().getSetting("my-greeting"), it works well.