Open the Extensions tab in VS Code by Ctrl + Shift + X and search raven.
From .vsix file
Head over to the Releases and download the release you want. In VS Code, press Ctrl + Shift + P to open commands, select Extensions: Install from VSIX... and browse for the .vsix file you just downloaded.
Building after hacking?
Just copy the entire folder and replace it with the one in {vscode_preferences_location}/extensions/.
How to create a .vsix package?
Install npm from your system marketplace.
Open your terminal and type npm install -g vsce.
Locate the theme folder in your system.
Enter vsce package and a .vsix file will be generated