How to customise the textMateRules for multiple themes in VS Code? [duplicate]

Question

In VS Code, I'd like to customise some textMateRules the same for multiple themes. For example, both for Atom One Dark and Default Dark+ but without affecting any of the other themes, I'd like to make the keywords italic. I can achieve this by duplicating the same settings twice separately for each theme as below

  "editor.tokenColorCustomizations": {
    "[Atom One Dark]": {
      "textMateRules": [
        {
          "scope": [ "keyword" ],
          "settings": { "fontStyle": "italic" }
        }
      ]
    },
    "[Default Dark+]": {
      "textMateRules": [
        {
          "scope": [ "keyword" ],
          "settings": { "fontStyle": "italic" }
        }
      ]
    }
  }

How can I only need to set up once for both, without duplicating the rules, especially if there are a lot same rules for the multiple themes? Something like the below (but which doesn't work though)

  "editor.tokenColorCustomizations": {
    "[Atom One Dark] [Default Dark+]": {
      "textMateRules": [
        {
          "scope": [ "keyword" ],
          "settings": { "fontStyle": "italic" }
        }
      ]
    }
  }