unfortunately, the media query does not affect the display in the Outlook app (version 4.7.0) on iOS. There is an option in preferences in the app itself to enable "Light" mode again (Settings - Appearance - "Light" instead of "System Default").
In fact, only a few apps do support the media query you mentioned (for example the native mail client of iOS - this works only if there is no accessibility feature like "Smart Invert" or "Classic Invert" enabled - and Edison Mail).
Our testing results showed that only Outlook, Edison Mail, Mail.ru, Yandex.ru and QQ Mail are changing the colors if dark mode is active. In Outlook, Edison Mail and Mail.ru it's possible to change the display mode to "light" in preferences of the apps.
Test device: iPhone 7, iOS 13.1.2
Apps tested: iOS native mail, Gmail, Outlook, Yahoo, AOL, GMX, Web.de, Freenet.de, Blue Mail, myMail, Spark, Edison Mail, Mail.ru, Yandex.ru, Interia.pl, O2.pl, WP.pl, email.cz, QQ Mail