Visual Studio Code is a go to application for coder's because of its rich theme and extension base. It's open source edition VScodium is also very popular among developers.
Open VSX Registry: A Vendor Neutral, Open Source Marketplace for VS Code Extensions. Where you can browse and download extensions.
For demonstration purpose I will install an extension called Prettier - Code formatter
Lets search and open Prettier - Code formatter from https://open-vsx.org/
After download open terminal and change the directory where you saved the extension file. By default it will be in Downloads folder.
If the file on that directory then simply run this command. You have to replace
<filename> with the filename of the extension that you've downloaded.
code --install-extension <filename>.vsix