Fix dark mode when system settings are set to light

[simon-retsch]

The recent fix (#175) for the problems with the dark mode did not resolve all problems.
I'm using Windows 11 which has also has the system wide dark mode settings. If the system wide dark mode is enabled it shows the entries correctly. But if the system dark mode is on light and the wagtail settings are on dark mode, it still shows the entries as described in here.