When comparing adapta-gtk-theme vs phlat, the Slant community recommends adapta-gtk-theme for most people. In the question“What are the best gtk3 themes?” adapta-gtk-theme is ranked 1st while phlat is ranked 4th.
Ranked in these QuestionsQuestion Ranking
Pro Follows the material design guidelines
Pro Most complete theme
It styles every nook and cranny of the OS UI, even the ugly Java apps look beautiful using this theme.
Pro Easy installation on Ubuntu
sudo add-apt-repository ppa:tista/adapta
sudo apt-get update
sudo apt-get install adapta-gtk-theme
Pro Real dark theme
Not only looks good, but really accelerates work on desktop.
Pro Fully supports GTK3-nocsd
The theme fully supports the GTK3-nocsd hack so GNOME3-apps will integrate well into XFCE or MATE.
Pro The color can be easily changed
A script is included to change the highlight-color to almost everything.
Pro GTK2 and GTK3
Supports both GTK2 and GTK3.
Pro Has full HiDPI support
The theme and all included sub-themes have full HiDPI support.
Con No debian support
Con Dark theme
It's a dark theme, so you might encounter the same issues as with all dark themes
Con No-GNOME etc. support
As it is made to support traditional desktop environments so there is no support for GNOME3 and its forks