Unfortunately, this is not possible. The Windows version of Opera has a feature in Settings > Browser > Show system color on top bar (http://www-static-blogs.operacdn.com/multi/wp-content/uploads/sites/3/2016/01/color-off-and-on.png), but this is not the case for Mac; probably because the dock and menu bar do not have as much customization. I would however love a feature to change the color to the tab bar, because it creates more contrast. That, and a menu button like in Windows, I think Opera would be golden.
So you are new to Opera. The major reason why I prefer this browser over all others is because I feel it has the best organization. All the managers are right on the side of the speed dial (customize your start page to show all), and the Bookmarks manager comes very handy rather than to have them all in folders on your Bookmarks bar.