1. Generate a VS Code theme extension project with VS Code's extension generator CLI tool. Follow the steps here: Create a new Color Theme
2. Download the JSON file for your theme. Go to Your Themes and hover over the theme you want to publish. Click the download icon that appears.
3. Replace the JSON contents of the generated theme file with the new JSON that you just downloaded.
4. Create a publisher and a personal access token. Follow these steps: Publishing an Extension
5. Add additional fields to your project's package.json file to improve your marketplace presence, and make it easier for users to understand what your extension is about. This includes things like adding an icon to your extension. Here are the fields that are available to you: Extension Manifest
6. Improve your extensions Readme.md file. This is a markdown file which displays information about your theme to the user when they click on it in the marketplace. Here are some tips on optimizing your Readme for the marketplace: Marketplace Presentation Tips
7. Publish your theme! Use the vsce CLI tool to do this easily. Follow the steps here: Publishing an Extension