When comparing viva vs Zukitre, the Slant community recommends Zukitre for most people. In the question“What are the best Gnome Shell themes?” Zukitre is ranked 8th while viva is ranked 12th. The most important reason people chose Zukitre is:
The design of Zukitre differs from Zukitwo in that there are fewer gradients and rounded corners. It is also designed to be more flat.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Available in Blue and Orange themes
viva offers a blue and an orange them so users can pick how the theme looks.
Pro Designed to be flat with sharp corners
The design of Zukitre differs from Zukitwo in that there are fewer gradients and rounded corners. It is also designed to be more flat.
Cons
Con Not finished
The them is not 100% finished yet, though the author does say he is working on it. Of course the latest commits can be seen on the github page with the last being from Sept. 2014.
Con Only supports the latest release of Gnome Shell and GTK
For Debian or Ubuntu users this theme will not work as it only supports the latest version of Gnome Shell and GTK.