[Safari] Mouse button 4/5 to Back/Forward

karabiner.json

{
  "description": "[Safari] Mouse button 4/5 to Back/Forward",
  "manipulators": [
    {
      "type": "basic",
      "from": {
        "pointing_button": "button4"
      },
      "to": [
        {
          "repeat": false,
          "key_code": "open_bracket",
          "modifiers": [
            "left_command"
          ]
        }
      ],
      "conditions": [
        {
          "type": "frontmost_application_if",
          "bundle_identifiers": [
            "^com\\.apple\\.Safari$"
          ]
        }
      ]
    },
    {
      "type": "basic",
      "from": {
        "pointing_button": "button5"
      },
      "to": [
        {
          "repeat": false,
          "key_code": "close_bracket",
          "modifiers": [
            "left_command"
          ]
        }
      ],
      "conditions": [
        {
          "type": "frontmost_application_if",
          "bundle_identifiers": [
            "^com\\.apple\\.Safari$"
          ]
        }
      ]
    }
  ]
}