MSN was tightly integrated into Windows 95, which had an MSN icon on the desktop. That didn't make MSN the "AOL-killer" that a lot of people (including some at Microsoft) were predicting.
Google is already paying nearly 80% of its AdSense revenues to partners. It's hard to see MSN being able to improve on that for any length of time.
Plus, Google doesn't reveal its compensation formula, and that makes it harder to cherry-pick its publisher base. What will probably happen is that unsuccessful Google publishers will try MSN (as some have tried YPN), leaving Google with the publishers who make good money for Google, advertisers, and themselves.
Another challenge will be building a critical mass of advertisers. Google has a huge head start, with ads for untold thousands of esoteric topics.
You don't need CSS to change ads in one fell swoop.