This example illustrates how you can transform your tokens stored on Figma Tokens (with GitHub sync enabled) to be automatically transformed with token-transformer and Style Dictionary to a Stitches environment with multiple themes. The theme switcher itself just adds light-theme or dark-theme to the root class, so in theory you could have not only light or dark theme but many different themes.
Change your tokens in tokens.json (either directly or with the Figma Tokens plugin in Figma). The GitHub action will automatically generate tokens to the tokens/ directory that can then be read by Style Dictionary, which will output tokens to the format you defined in build.js - css variables will be generated in the output directory which Stitches will pick up.
Note: Ideally generated css/dist files wouldn't be part of the repository but rather live in the build environment. I only included them here for showcase.