Refer here.

There are several ways to make it(by “dconf-editor” or “dconf-cli”). The basic concept is to change Unity configuration with “dconf“.

In “dconf-editor”, open “com/canonical/desktop/interface” from the left tree view, and change “scollbar-mode” to “normal”, done.

With “dconf-cli”, run the following command in terminal:

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gsettings set com.canonical.desktop.interface scrollbar-mode normal

Note that the key point is that GNOME uses dconf to save configurations.