Depends on the guidance you're looking for. The primary reason everything was battleship grey in Winforms was less because the Microsoft guide said it should be (it didn't) and more because that was the default and it was a pain to write it differently. Even now, I would imagine that the bulk of the LOB apps written with Silverlight or WPF will use default colors and styles for exactly the same reasons.
But a lot of the other UI guidelines can still apply. If you want something the looks and feels familiar, there's no reason that you can't make a standard menu bar with File, Edit, View, Help, etc. You can still use the same hotkeys, same commands, same layout for buttons and controls.
Keep in mind though that these guidelines were written with assumptions about software and computers in general that are no longer true. The dominant paradigm has changed and people are far more used to websites with different UI layouts and richer visuals. As a result, visual style is a lot more diverse and people are less likely to be confused by some non-standard layouts and controls. Which doesn't mean that anything goes, just that we should feel less contrained to keeping things in the exact same order and position, lest our customers freak out because they can't find the save button.
In short, the style guide was there because there wasn't enough for a real designer to do but still enough that we developers could make things ugly. Now it's even easier to make really ugly stuff, but there's a lot that a real designer can do to make it nice. So hire one. It's worth it.