In version 9.6 of WebMail Lite we have added Dark theme which can be selected from the list of themes in account settings.

It can also be selected as a default one in admin interface of WebMail Lite installation:

Dark theme isn't in the list of themes

If you have upgraded from previous version of WebMail Lite the new theme wouldn't be listed there indeed, you'll need to adjust the product configuration.

Modify /data/settings/modules/CoreWebclient.config.json configuration file and add "DefaultDark" entry under "ThemeList":

    "ThemeList": [
        [
            "Default",
            "DefaultDark",
            "DeepForest",
            "Funny",
            "Sand"
        ],
        "array",
        null,
        "List of themes available"
    ], 

I still see white background

The absolute majority of email messages these days are HTML, and they have their own background. WebMail Lite keeps the original background color of the emails. Currently, there's no good way to automatically recolor arbitrary email messages to make them Dark theme compliant, if such a method becomes available in the future we'll consider adopting it.

That's also why we've kept white background on composing mails, so they show in exact same way upon viewing. For unifications reasons, plaintext messages are shown with white background, too.