One step:
I am going to show you the installation of a theme with Numix theme and Unity Tweak Tool. If you is 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'll so easy to use it
Two-Step:
I must download themes
So Ican access:https://www.gnome-look.org/
The website has so much themes, IT support gnome,but also support unity
So, 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:
Need to extract (unzip):
Use Comment:
sudo tar-xzvf ... tar.xz
The next step:you need copy it to/usr/share/themes/
So, 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 what to install themes
Ubuntu Install themes && use it