Examine source code of Hmmmm......

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "bla",
  "theme": {
    "images": {
      "additional_backgrounds": [
        "images/bg-032-c7f80ac9e21af771c8d088973b1309ea.svg"
      ]
    },
    "properties": {
      "additional_backgrounds_alignment": [
        "top"
      ],
      "additional_backgrounds_tiling": [
        "repeat"
      ]
    },
    "colors": {
      "toolbar": "rgb(0, 0, 0)",
      "toolbar_text": "rgb(255, 255, 255)",
      "frame": "rgb(35, 35, 36)",
      "tab_background_text": "rgb(255, 255, 255)",
      "toolbar_field": "rgb(255, 255, 255)",
      "toolbar_field_text": "rgb(0, 0, 0)",
      "tab_line": "rgb(255, 255, 255)",
      "popup": "rgb(0, 0, 0)",
      "popup_text": "rgb(255, 255, 255)",
      "button_background_active": "rgb(35, 35, 36)",
      "button_background_hover": "rgb(76, 76, 78)",
      "icons_attention": "rgb(255, 255, 255)",
      "icons": "rgb(255, 255, 255)",
      "ntp_background": "rgb(0, 0, 0)",
      "ntp_text": "rgb(255, 255, 255)",
      "popup_border": "rgb(0, 0, 0)",
      "sidebar_text": "rgb(0, 0, 0)",
      "tab_loading": "rgb(0, 0, 0)",
      "tab_selected": "rgb(35, 35, 36)",
      "tab_text": "rgb(255, 255, 255)",
      "toolbar_field_border_focus": "rgb(0, 0, 0)",
      "toolbar_top_separator": "rgb(124, 124, 127)"
    }
  }
}