Outlook for Mac modifies all colours in dark mode to ensure all emails meet Microsoft’s specific targets for darkness and contrast.
This modification in dark mode is a built-in feature of Outlook for Mac that will run whether you have set custom dark mode styles using dark mode editing or not.
If you have not set up custom dark mode styles in your email, Outlook for Mac will simply modify your light mode colours to make them all dark. As you can imagine, this can result in some very big shifts, for example when changing light or white backgrounds to be very dark.
If you have used dark mode editing to configure custom dark mode styles in your email, your colours will still subject to modification by this process if
They are considered too bright
There is not enough contrast between the colour and any text over it.
If you use a dark colour palette for dark mode, you are unlikely to see any difference between the results in Apple Mail and the results in Outlook for Mac.
If however you try to use bright or light colours in your dark mode colour scheme, those colours are likely to be adjusted in Outlook for Mac in dark mode.
When using dark mode editing, you can prevent modifications from happening if you do the following when selecting your dark mode colours:
Avoid bright colours
Avoid bright background colours, even for buttons
Avoid white backgrounds, even for buttons.
By selecting darker colours, you can ensure that your colour choices will not be modified by Outlook for Mac in Dark Mode when it renders your email.