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I am assuming you are declaring inline styles rathar than global CSS for emails. The latter goes against all best practices in the widest context, not just Marketo.
I am also assuming you are declaring something like-webkit-font-smoothing:antialiased;-webkit-text-size-adjust:none;
That is often used for responsive emails.
Outlook is just highlighting its limitations. That is unrelated to Marketo in general and Design Studio in particular.
Marketo will send the code as you defined it. It is up to the email clients to render what they receive. Outlook up to 2007 used a proper rendering engine. It switched to MS-Word (yes, a word processor) as rendering engine from 2010.
You can find the capabilities and limitations of popular email clients at