Option for flat styled windows

[notanimposter]

Many web apps have colored toolbars at the top of the page. It would look much nicer if, for these sites, there was an option to have the titlebar be "flat" so it blends in with the app. Here is an example of what it looks like now:

And here is what it could look like if such an option were implemented:

To accomplish this style, the headerbar must have the "flat" CSS class and a background color. Whether to render the app as "flat" would be chosen in the Webpin edit screen, along with "Use minimal UI", etc.