Why is syntax highlighting not showing for my custom extension in VS Code? - visual-studio-code

Question

I am trying to create a custom language-extension for VS Code. I was mostly following the instructions given here and here.
When I run the extension in the Extension Development Host by hitting F5, everything works fine. As soon as I try to install my extension and use it in VS Code, it does no longer.
From here I gathered that I have three options of installing my extension
bundle my extension to a vsix with vsce and install it from within VS Code
install the vsix package from command line with code --install-extension myExt.vsix
copy the content to a folder in %userprofile%\.vscode\extensions
All three options give the same result: syntax highlighting does not work for keywords, etc. but it does work for comments. Using the Scope Inspector I can see that all the tokens are correctly deteced and categorized, so the extension seems to be basically working, but code is not highlighted (again, except for comments, weirdly).
The only clue I get when I install via command line is
Installing extensions...
(node:34352) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.
Extension 'vscode-knitout.vsix' was successfully installed.
but I doubt that this has got anything to do with the problem.
This here is my package.json file:
{
"name": "vscode-myLanguage",
"displayName": "myLanguage",
"description": "myLanguage language extension for VS Code",
"version": "0.0.1",
"publisher": "me",
"author": {
"name": "name"
},
"icon": "images/icon.png",
"galleryBanner": {
"color": "#C80000",
"theme": "dark"
},
"engines": {
"vscode": "^1.53.0"
},
"license": "SEE LICENSE IN LICENSE.txt",
"repository": {
"type": "git",
"url": "https://some.git.url"
},
"homepage": "https://our.website",
"categories": [
"Programming Languages"
],
"contributes": {
"languages": [{
"id": "myLanguage",
"aliases": ["myLanguage", "myLanguage"],
"extensions": [".m",".myLanguage"],
"configuration": "./language-configuration.json"
}],
"grammars": [{
"language": "myLanguage",
"scopeName": "source.myLanguage",
"path": "./syntaxes/myLanguage.tmLanguage.json"
}],
"themes": [
{
"label": "myLanguage-dark",
"uiTheme": "vs-dark",
"path": "./themes/myLanguage-vs-dark-color-theme.json"
}
]
}
}
When I run via Extension Development Host, the Scope Inspector correctly recognizes the language and textmate scopes and respective theme rule for 'foreground'. When I run VS Code with the extension installed/copied, the Scope Inspector says "No theme selector" for 'foreground', but everything else (scope, language) looks correct.
What is the problem here? Is there a very simple example somewhere for a reference?