I am an independent advisor answering Outlook questions.
First, let me say, you are the first person I encountered who provided a good review about the New Outlook other than me. It has potential, but I use Outlook Web 98% of my time. I am glad the Classic Outlook is being revamped.
Currently, you cannot change a message's font size and color based on criteria. It took me a while to "get over missing" this feature during my transition from classic to online. As a workaround, you can create a rule to pin to top and assign a category color.
Also, Microsoft is requesting feedback from users. That's the beauty of being an early adopter - Influence Change.
* In New Outlook, select Help | Feedback.
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