One of the Google AdSense optimization tips says to, "Use a CSS form to embed the ads unit for more seamless design." What does this mean? How does one implement a CSS form?
It's a typo (or it's badly phrased). There's no such thing as a "CSS form". What they mean is this:
"Use the CSS form to embed the ad unit for [a] more seamless design."
When you create an ad unit in the AdSense control panel, there's an "Ad style" form that lets you control the CSS styles like the one pictured below. This is probably what they're referring to. Changing these values to affect the style of the AdSense block is likely to have an effect on your revenue.
Confusingly, Google suggest that both making ads stand out and making ads blend in will boost your AdSense revenue, so it's probably best to experiment with both and form your own conclusion based on your site design and audience.