Flattiance is pitched as a “semi-flat fork” of the Ubuntu Ambiance theme. You know, the one that ships out of the box and by default.
On the whole Flattiance keeps to the same color palette, with dark browns and orange accents, but it ditches the gradient in app headers in favour of a solid block.
The window control icons are the biggest divergence from the theme it mimics, though. The bright traffic light colours are more akin to macOS than anything we’re used in on Ubuntu.
You can download the theme from the link below. To install just extract the downloaded archive to your ~/.themes folder and then use a utility like Unity Tweak Tool to set it.