When Google redesigned Gmail a couple of years ago, the new look also came with new scrollbars. Not everyone liked the change, but for those of us who do, the latest version of Chrome (31) has a treat for us.
First, an explanation of just what the overlay in overlay scrollbar means. Simply put, traditional scrollbars need their own space to operate, meaning that the web page you're viewing is butted up against the scrollbar. An overlay scrollbar lays on top of the content, and therefore doesn't require its own space. The page you're viewing takes up the entire browser window. Less chrome, more content- it's a design trend that I sincerely hope continues to gain momentum. In the screnshot above, you can see just how stark the difference is. Note that the new scrollbar expands when you're scrolling, or when you hover over it, then shrinks again when you don't need it.
If you want the new overlay scrollbars, you don't need an extension, or even a userstyle. Just head to chrome://flags in Chrome's omnibox, hit ctrl+f to search, and type "overlay." You'll find the entry, aptly enough, under "overlay scrollbars." Click to enable, then relaunch Chrome by pressing the "relaunch" button at the bottom of the page.
That's it. Now you'll have less intrusive scrollbars on every page. I should note that, to this point, the Minimal Scrollbars extension has been my scrollbar of choice. Like most other scrollbar extensions, though, it's not perfectly consistent, and doesn't work on some pages. Chrome's built-in scrollbars work everywhere, creating a more consistent experience. That said, if you don't like Gmail-style scrollbars, I highly recommend the Minimal Scrollbar extension.