Install latest/stable of gtk-theme-traditionalhumanized
Ubuntu 16.04 or later?
Make sure snap support is enabled in your Desktop store.
Install using the command line
sudo snap install gtk-theme-traditionalhumanized
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This snap provides the GTK themes for Ubuntu MATE in a form that can be used by other snapped applications. It can be installed alongside the gtk-common-themes snap.
Generate an embeddable card to be shared on external websites.
Choose your Linux distribution to get detailed installation instructions. If yours is not shown, get more details on the installing snapd documentation.