Are you still looking for best flat theme for your desktop? Here is Flatabulous theme which claims to be the best flat theme derived from ultra-flat theme released under The MIT License, so the hierarchy of this theme is like Flatabulous derived from ultra-flat and ultra-flat theme is modified version of Numix theme, so credit also goes back to Numix theme. You can guess the look of the theme by its name, says it's flat theme. This theme has dark title bars with flat rounded window color buttons and grey color selection. It is specially made compatible with Unity desktop but it can also work in other desktops too such as Gnome, Xfce and so, screenshots are displayed below and shadow icons used in screenshots. Since this theme is in active development mode, if you experience any kind of issue while using it then report to get it fixed. You can use Unity Tweak Tool, Gnome-tweak-tool or Ubuntu-Tweak to change themes.
