我有 Ubuntu 16.04，想安装 Adapta 深色主题，但我对 Linux 不太熟悉，所以请耐心等待。我尝试按照此页面上的说明进行操作在 Ubuntu 上安装 Adapta GTK 主题的指南，但是没有起作用，运行后我从终端得到了以下输出sudo apt update && sudo apt install adapta-gtk-theme：
Hit:1 http://la-mirrors.evowise.com/ubuntu xenial InRelease
Hit:2 http://la-mirrors.evowise.com/ubuntu xenial-updates InRelease
Hit:3 http://ppa.launchpad.net/dawidd0811/neofetch/ubuntu xenial InRelease
Hit:4 http://download.mono-project.com/repo/ubuntu xenial InRelease
Hit:5 http://la-mirrors.evowise.com/ubuntu xenial-backports InRelease
Hit:6 http://ppa.launchpad.net/jtaylor/keepass/ubuntu xenial InRelease
Hit:7 http://la-mirrors.evowise.com/ubuntu xenial-security InRelease
Hit:8 http://archive.canonical.com/ubuntu xenial InRelease
Hit:9 https://deb.opera.com/opera-stable stable InRelease
Hit:10 http://ppa.launchpad.net/mikhailnov/pulseeffects/ubuntu xenial InRelease
Hit:11 https://dl.winehq.org/wine-builds/ubuntu xenial InRelease
Hit:12 https://repo.skype.com/deb stable InRelease
Hit:13 http://ppa.launchpad.net/rebuntu16/avidemux+unofficial/ubuntu xenial InRelease
Hit:14 https://download.mono-project.com/repo/ubuntu stable-xenial InRelease
Hit:15 http://ppa.launchpad.net/tista/adapta/ubuntu xenial InRelease
Hit:16 http://ppa.launchpad.net/ubuntuhandbook1/sigil/ubuntu xenial InRelease
Hit:17 http://ppa.launchpad.net/vincent-c/wesnoth/ubuntu xenial InRelease
Hit:18 http://ppa.launchpad.net/webupd8team/java/ubuntu xenial InRelease
Reading package lists... Done
Building dependency tree
Reading state information... Done
40 packages can be upgraded. Run 'apt list --upgradable' to see them.
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package adapta-gtk-theme
我是不是漏掉了一些步骤？我已经安装了 Unity Tweak Tool。