update the markdown alert styles

[devoncarew]

• update the markdown alert css to a style closer to that used on dart.dev
• close update the css of the new github alerts feature #3627

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[devoncarew]

before:

after:

[srawlins]

Very nice!

[devoncarew]

@srawlins - separately, I noticed that the markdown blocks render pretty narrow - more narrow than the same type of content on dart.dev. We have the width limited currently to 700px:

We may want to expand that. I see that dart.dev is using a max-width of 960px for similar content.