I just seen yesterday that the Gmail App on my iPhone has already adapted the Dark Mode of phone's system. If your Gmail's theme setting is set into Default it will adapt depending on your Appearance settings. I've tried several fixes that exists around the email world including the media query and meta data workaround but didn't worked. What's interesting is, I've seen via Inspect Element that Gmail strips those defined CSS under the media query (prefers-color-scheme: dark).

Has anyone tried to bypass Gmail's validation of CSS? Could it be that the mentioned media query isn't still supported as of moment?