message: add white background and z-index to dropdown content
diff: {
  "name": "primer-react",
  - "version": "0.0.2-beta",
  + "version": "0.0.3-beta",
  "lockfileVersion": 1,
  "requires": true,
  "dependencies": {
  "es5-ext": "0.10.42"
  }
  },
  + "d3-path": {
  + "version": "1.0.5",
  + "resolved": "https://registry.npmjs.org/d3-path/-/d3-path-1.0.5.tgz",
  + "integrity": "sha1-JB6xhJvZ6egCHA0KeZ+KDo5EF2Q="
  + },
  + "d3-shape": {
  + "version": "1.2.0",
  + "resolved": "https://registry.npmjs.org/d3-shape/-/d3-shape-1.2.0.tgz",
  + "integrity": "sha1-RdAVOPBkuv0F6j1tLLdI/YxB93c=",
  + "requires": {
  + "d3-path": "1.0.5"
  + }
  + },
  "dashdash": {
  "version": "1.14.1",
  "resolved": "https://registry.npmjs.org/dashdash/-/dashdash-1.14.1.tgz",
  