one step:
I am going to show you the installation of a theme with Numix theme and Unity Tweak Tool. If you are using Gnome instead of Unity desktop environment, you can use Gnome Tweak Tool.
i use Unity ,so i install unity Tweak tool
sudo apt install unity-tweak-tool
after you open software
next, open it ,you will so easy use it
two-step:
i must download themes
so ican access: https://www.gnome-look.org/
the website have so much themes, it support gnome,but also support unity
so you can use this comment to download the package:
wget https://dl.opendesktop.org/api/files/download/id/1511520322/T4G-3.1-theme.tar.xz
three step:
you need to extract(解压)：
use comment:
sudo tar -xzvf .......tar.xz
the next step: you need copy it to /usr/share/themes/
so ,you can open unity-tweek-tool to choose it ,
ps: you extract tar.zx file ,you open the file ,you need to see ....md ,
the file tell you how to install themes