When a default theme is provided it is used in place of the standard "fl-theme-prefsEditor-default". However, there are specific styles applied to "fl-theme-prefsEditor-default" to properly style the colour and contrast adjuster option for it.
This manual test uses a custom default theme. Since it didn't supply the correct styling, the cross out effect is missing. Additionally the default option picks up the style of whichever theme is activated.