This guide will help you install an app called, "GNOME Software". In this guide "GNOME Software" will be refered to as, "GNOME’s appstore" for simplicity. The GNOME’s appstore is a really easy to use appstore by GNOME. GNOME’s appstore provides a simple UI for downloading and installing apps that use GNOME’s theme system. If you have followed the Dark Theme Guide apps installed from here should have a dark or custom theme by default.
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Step 1: Open the terminal and type in, "sudo apt-get upgrade -y" and press enter.
Tip: To paste commands into the terminal, just right-click the command line.
Step 2: Type in, "sudo apt install gnome-software -y" and press enter.
Step 3: Once it is done, open the laucher and click on an app called, "Software"
Step 4: Download some apps! Preferable download apps made by The GNOME Project, as they will have support for custom themes and are secure.