I've asked similar and was adviser to go and ask the developer of the theme, but I dont think the problem lies with them.
I've tried over a 100 themed, both brand new ones and old veterans and not one seems to have the dark mode used on the default email app. The SMS, keyboard, menu and calendar are all able to take the colour of the theme but not the default Samsung email app. This makes me now know that fault lies with Samsung and their OneUI.
So my question is, can Samsung please make it available for other themes to utilise the dark / night option on all your native apps, in particular, the email app.