Semantic highlighting does not appear to work (due to theming)

1. I expect the first four characters to be highlighted but nothing happens.
2. There is no documentation really for any of the functions but releaseDocumentSemanticTokens in particular is not clear to me what I should be doing there.

monaco.languages.register({
    id: 'new'
});
monaco.languages.registerDocumentSemanticTokensProvider('new', {
    getLegend: () => {
        return {
            tokenModifiers: [],
            tokenTypes: [
                "keyword"
            ]
        }
    },

    provideDocumentSemanticTokens: () => {
        return {
            data: [
                0, 0, 4, 0, 0
            ]
        }
    },

    releaseDocumentSemanticTokens: () => {
    }
});

monaco.editor.create(document.getElementById("container"), {
    value: "12345678",
    language: "new"
});