By default, Ubuntu 18.04 LTS will only use GNOME Shell desktop, there is not any way to change your desktop theme. However, if you are looking for a bright theme or a nice dark theme for your Ubuntu LTS desktop, here’s how you can customize it.
Although there are not many customization options, customizing the GNOME Shell desktop is very simple.
How to Change Desktop Themes
If you want to change your desktop theme, you should install the GNOME Tweaks app, previously known as GNOME Tweak Tool. To install it, you simply open the Ubuntu Software app, search and then install GNOME Tweaks application. If you want to install GNOME Tweaks from terminal, just the following command:
sudo apt install gnome-tweaks
Once you have installed the app, you can open the “Tweaks” shortcut from the desktop’s app menu. In the Tweaks window, simply click on the “Appearance” section and under Themes, use the options to change your theme settings.
By default, it’s “Ambiance” theme for Applications, DMZ-White theme for the mouse cursor, as well as Ubuntu-mono-dark icon theme. While the Shell theme option is not handy, you can control the style of the desktop shell.
From there, you can also enable the Adwaita icon theme, although the Adwaita icons are incomplete. In order to install the full Adwaita icon theme, launch a terminal window and run the following command. Next, when you are prompted, enter your password:
sudo apt install adwaita-icon-theme-full
How to Install Popular Arc Theme
Arc is another popular Linux GTK theme. It’s a flat theme featuring some transparent elements and using blue accents instead of the original orange accents of Ubuntu. It supports for both light and dark variants. To install the Arc theme, just launch a Terminal window and then run the following command. Once done, when you are prompted, enter your password and type “Y” to confirm your choice:
sudo apt install arc-theme
Now, you can select the Arc, Arc-Dark, or Arc-Darker themes in the Tweaks app. When the Tweaks app has already been running, you will have to close and then reopen it after installing a new theme.
The Arc theme is blue and white, while the Arc-Darker theme is blue and dark gray. The last one is the standard blue and white theme, but there are dark gray title bars and sidebars.
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How to Install More Themes
There are still multiple ways to install more themes, which can be installed from Ubuntu’s software repositories. You can also get them from a package archive (PPA), or download .deb packages containing themes, or install extracted themes from .zip or .tar.gz files manually.
You can also search for lists of Ubuntu themes on the internet and then you can decide which ones you want to install. Each theme will also give you a detailed instruction for the installation. You can find it in its README file or on the download page. Keep in mind that you should follow these instructions carefully, as some themes will require additional software or more installation steps.