Examine source code of LibreOffice Vanilla

{
  "theme": {
    "images": {
      "theme_frame": "persheader.png"
    },
    "colors": {
      "frame": "rgba(229,230,232,1)",
      "tab_background_text": "#000"
    }
  },
  "version": "2.0",
  "name": "LibreOffice Vanilla",
  "manifest_version": 2,
  "description": "Gradient taken from LibreOffice interface, to create a seamless top menu bar in LibreOffice."
}