How to change messages to light mode when theme is set to dark mode

Dear opossumolivia,

Good day! Thank you for posting to Microsoft Community. We are happy to assist you.

As per the description you have shared, we understand that you have a concern with Outlook theme color in the New Outlook for Windows.

We have been testing your query from our end and we are also experiencing same issue as on your end. Yes, you are definitely right, in classic Outlook there is an option to choose Dark Gray to lighten message list and reading pane. You can refer to the screenshot below.

However, in New Outlook for windows, there is only Dark, Light and use system setting options. Typically, when you select Dark mode in Outlook, as you mentioned, a user must toggle on the sun button to change it to light mode for each individual messages. Therefore, we do understand your query here and we do apologize not able to meet your requirement at this moment. However, thank you for trying New Outlook for Windows and your valuable suggestion. At this point, we will surely raise your concern to our Product Development Team and hopefully this can be included in the future updates. In the meantime, we sincerely recommend you post your valuable suggestion as an idea on our in-app New Outlook feedback site. To provide feedback, go to Help> Feedback. You can refer to Contact support and provide feedback within the new Outlook for Windows - Microsoft Support

Our Product Development Team are committed to enhancing the customer experience by continually examining suggestions to include in the design and development of future product upgrades based on your suggestions. Hopefully your suggestion will help many users in the future.

Thank you for your precious time, valuable suggestion and understanding. For other concerns, please do not hesitate to add your post in the Microsoft Community Team.  

Stay safe and healthy. Have a nice day!

Sincerely,

Libeamlak | Microsoft Community Moderator