Tweaks made to achieve this https://twitter.com/jay_wick/status/995636552905584640

custom-vscode.css

/*
   credit @mikewheaton's code plus some small tweaks
   https://github.com/open-source-ideas/open-source-ideas/issues/10#issuecomment-272638529 
*/

.mtki {
  font-family: 'Ubuntu Mono' !important;
  font-style: italic;
  font-size: 1.2em; /* Lines up perfectly with Fira Code */
}

user-settings.json5

// install https://github.com/be5invis/vscode-custom-css
{
  "editor.fontFamily": "Fira Code",
  "editor.fontLigatures": true,
  "workbench.colorTheme": "Pure Code",
  "editor.fontSize": 15,
  "editor.lineHeight": 22,
  "vscode_custom_css.imports": [
      "file:///PATH_TO_YOUR_CUSTOM_CSS/custom-vscode.css"
  ],
    "editor.tokenColorCustomizations": {
      "[Pure Code]": {
          "textMateRules": [
              {
                  "scope": "keyword.control",
                  "settings": {
                      "fontStyle": ""
                  }
              },
              {
                  "scope": "keyword.operator",
                  "settings": {
                      "fontStyle": "",
                  }
              },
              {
                  "scope": "variable.parameter",
                  "settings": {
                      "fontStyle": "italic"
                  }
              },
              {
                  "scope": "entity.name.function",
                  "settings": {
                      "fontStyle": "italic",
                      "foreground": "#ececec"
                  }
              }
              ,{
                  "scope": "string,punctuation.definition.string",
                  "settings": {
                      "foreground": "#e7b6d1"
                  }
              },
              {
                  "scope": "string.template",
                  "settings": {
                      "foreground": "#e9d7e1",
                  }
              }
          ]
      }
  },
}