When comparing Arc vs adapta-gtk-theme, the Slant community recommends Arc for most people. In the question“What are the best Gnome Shell themes?” Arc is ranked 1st while adapta-gtk-theme is ranked 2nd.
Ranked in these QuestionsQuestion Ranking
Pro Customizable to dark or light themes
Pro Has both GTK2 and GTK 3 support
Users will be able to match their gtk2 and gtk3 themes easily due to Ark supporting both.
Pro Supports gnome-shell
Pro Available firefox and chromium themes
There are matching themes for Firefox and Chromium for those that want to have a fully matched looking environment.
Pro Really clean and nice-looking
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.
Con Bleak font colors that cause eye strain
The font color in Arc and Arc-Darker is not black, but lightgray
after prolonged usage, it becomes apparent that this font color, combined with white backgrounds causes too much eye strain
Con Has some contrast issues
The theme is a low contrast theme so it really depends on the screen brightness if you see borders or the text, some elements don't even have borders - this makes the theme barely usable for portable devices.
Con No debian support