Apparently, night mode with the samsung email widget does not work. This is very disappointing due to the fact that this Is a system app and it doesnt support night mode. Meanwhile, the samsung music app widget supports night mode...C'mon Samsung, fix this issue
You can change the colour and transparency of the widget manually. Just remove it and add it back. In fact there seems to be a toggle for the widget to follow the day/night mode, but it (rightly) restricts the transparency setting so that the widget doesn't suddenly become unreadable.