In a terminal window, you may install many themes from Ubuntu’s software repositories. Using Archive Manager, some themes must be manually extracted. GNOME Tweaks must be downloaded and installed from the Ubuntu Software Center in order to apply installed themes in Ubuntu.
Steps to install theme
Step 1: Search for a theme
Step 2: Open terminal and type this command
Install theme package:
sudo apt-get install [package name]-theme
Install an icons theme:
sudo apt-get install [package name]-icons
Step 1: Search for a theme:
Step 2: Download desired themes in zip file
Step 3: Navigate the downloaded zip and extract the theme.
Step 4: Copy the extracted folder
.themes folder in ~/.themes for themes
.icons folder in ~/icons for icons
Here are some of the available themes in ubuntu .
Orchis GTK Theme