When comparing Paper vs Zukitwo, the Slant community recommends Zukitwo for most people. In the question“What are the best gtk3 themes?” Zukitwo is ranked 16th while Paper is ranked 19th. The most important reason people chose Zukitwo is:
Zukitwo theme offers a matching GTK theme to the built in Gnome Shell theme.
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Pros
Pro Flat and colorful design
The look of this theme is inspired by Material Design.
Pro Does total makeover of your computer
It's a complete theme, with custom icons, GTK, cursors, Gnome Shell.
Pro Built in matching GTK theme
Zukitwo theme offers a matching GTK theme to the built in Gnome Shell theme.
Pro Offers PPA for easy install on Ubuntu based distros
Zukitwo offers an Ubuntu PPA for Ubuntu based distros.
sudo add-apt-repository ppa:noobslab/themes
sudo apt-get update
sudo apt-get install zukitwo
Cons
Con There is jumpy behavior when switching from window to window
You can notice jumpy behavior when switching from a forefront window to window behind it. Both windows jump a little which can slightly disrupt normal working flow.
Con Only supports latest version of Gnome Shell and GTK
Zukitwo only officially supports the latest release of Gnome Shell and GTK.