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(解壓)：

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

ubuntu install themes && use it