This is pretty straight forward. I using your theme with the gatsby-remark-vscode plugin for GatsbyJS.
It works locally, but when I try to deploy the site to Netlify, I get the following error:

I guess their build step is case-sensitive. In your package.json:
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{ |
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"label": "Morass (rainglow)", |
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"uiTheme": "vs-dark", |
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"path": "./themes/morass.json" |
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}, |
But your filename inside theme is Morass.json
Thanks!