wp_customize_support_script()

wp_customize_support_script() — Prints a script to check whether or not the Customizer is supported, and apply either the no-customize-support or customize-support class to the body. This function MUST be called inside the body tag. Ideally, call this function immediately after the body tag is opened. This prevents a flash of unstyled content. It is also recommended that you add the “no-customize-support” class to the body tag by default.