Oomox is a tool that allows generating color variations of the popular Numix GTK2 / GTK3 theme, as well as icons to match them.

The application comes with a large number of built-in presets that can be further customized, and it supports GNOME, Unity, Xfce4 and Openbox.

The minimum GTK3 version required to use it is 3.18 (so it supports Ubuntu 16.04 and newer). The themes might work with older GTK3 versions, but you may encounter issues.

Oomox already had Unity support, but with the latest 1.2.0 version, themes generated with it now have the correct window buttons color instead of the default Numix color:

Oomox is a tool that allows creating different color variations of the popular Numix GTK2/GTK3 theme. It features built-in presets, so you can easily generate various themes without much effort, or you can use the tool to change the theme colors individually.

In Ubuntu 15.10 (which uses GTK 3.16), there's a bug which causes some themes, like Numix, to have large black borders.

This bug only occurs for applications that use header bars (client-side decorations) and it affects Ubuntu (with Unity), while Xubuntu, Ubuntu MATE and Ubuntu GNOME don't seem to be affected (at least in my quick test). Update: MATE with Compiz enabled is also affected so I guess any desktop environment under which you use Compiz is affected (thanks to pawelp!).

Here's a screenshot with GNOME Clocks (an app that uses header bars) under Unity (Ubuntu 15.10), using Numix GTK theme:

Ideally, you'll need a complete GNOME 3.14 / 3.12 desktop to see what Paper is all about, since the theme uses custom colors for header bars / client-side decoration apps.

Ubuntu GNOME with GTK/GNOME Shell 3.12: if you're an Ubuntu GNOME user, you're most probably aware that Ambiance and Radiance (the default Ubuntu themes) don't support GNOME Shell and applications that use header bars look broken. Well, Leo Iannacone has fixed the default Ubuntu themes to work properly under GNOME 3.12.

Here are a couple of screenshots taken under GNOME Shell / GTK+ 3.12 (Ubuntu 14.10 Utopic Unicorn) with fixed Ambiance and Radiance themes, using proper header bars (client side decorations):

Unfortunately, Leo's fixed Ubuntu themes don't work properly with Unity. For instance, the Nautilus toolbar/pathbar looks pretty weird. So I recommend using them only with GNOME Shell (Ubuntu GNOME).