Auto change html attributes on page load

I am trying to create a custom google chrome theme (image displayed in a new tab). However, there are these last recently visited blocks blocking part of the image and trying as best I could, I cannot find an extension or theme on the internet that can get rid of them. I know what changes need to be made to achieve this; The HTML page contains the following line:

<div id="mv-tiles" style="opacity: 1;"> == $0

      

        
        
        
      

    

and I just need to change style

to "opacity: 0;"

. I could do this with static HTML from Excel (using getElementById("mv-tiles").style = "opacity: 0;"

or something like that)

But how do I do it dynamically, whenever I open a new tab, not from Excel, just with my .JSON file from a chrome theme or some addon?

Update

I just pasted the provided code into my JSON file as shown below - I then created the file in notepad where I pasted the JS code, saved it as content.js

. Finally, I dragged the entire folder into the chrome extensions page to save it as a .crx package and then install. Here's my finished manifest.json

file:

{
  "manifest_version": 2,
  "permissions": [
    "<all_urls>"
  ],
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_end"
    }
  ],
  "theme": {
    "images": [],
    "colors": {
      "frame": [
        66,
        116,
        201
      ],
      "toolbar": [
        223,
        223,
        223
      ],
      "tab_text": [
        0,
        0,
        0
      ],
      "tab_background_text": [
        64,
        64,
        64
      ],
      "bookmark_text": [
        18,
        50,
        114
      ],
      "ntp_background": [
        255,
        255,
        255
      ],
      "ntp_text": [
        0,
        0,
        0
      ],
      "button_background": [
        0,
        0,
        0,
        0
      ]
    },
    "tints": {
      "buttons": [
        0,
        0,
        0.5
      ]
    },
    "properties": {
      "ntp_background_alignment": "bottom",
      "ntp_background_repeat": "no-repeat"
    }
  },
  "name": "Test Theme",
  "version": "1",
  "description": "Removes thumbnails"
}

      

        
        
        
      

    

It doesn't work as expected (the theme looks great, but the JS won't start!)