So if you wanted to see the Ruby colors, for example, those are in src/partials/ruby.js.
Those files are imported, merged, and written out to themes/dark-plus-syntax-color-theme.json
To make changes
Clone the project
Change a file in src/partials
Run npm install to install development dependencies.
Run npm run build to build the theme file with the new changes.
Go to the Debug dropdown and click Start Debugging to run the new version of the theme locally.