@ChielD1975 , Based on my checking in my Intune portal, I did not find the policy to change the message font and color in outlook under device configuration profile. To accomplish this, I think we can use script to do this.
Research and find a link include the script, we can read it as a reference:
https://stackoverflow.com/questions/58182253/modifying-powershell-script-to-change-the-default-font-type-and-size-for-outlook
Note: Non-Microsoft link, just for the reference.
Hope it can help.
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