I changed a few items in v11 by creating an App_Plugin and adding a few lines to the appsettings.json file. With a little exploring you can find the css classes you want to change.

1. Create a folder in App_Plugins. Add your css file and the package.manifest so it points to your css file. My css file looks like this:

   .login-overlay { background-color: #fff; }

   .login-overlay__background-image { background-color: #fff; background-size: cover; }

   div.login-overlay__logo { height: auto; max-width: 240px; width: 20%; }

2. Edit your appsettings.json file. Add the lines below to the Umbraco/CMS/Content node. Edit to point to your files.

   "LoginBackgroundImage": "../images/background.jpg", "LoginLogoImage": "../images/logo.png"

Hopefully that gets you started in the right direction.

You are correct. I was thinking of the login page.

If it helps anyone I managed to do this easily in U13 by creating folder App_Plugins/BackOfficeTheme and adding an index.css and package.manifest file as follows:

#umbraco-logo-mark{
    background-image: url('/images/core/codebricks_128.png');
    background-size:cover;
}
#umbraco-logo-mark svg {
    opacity: 0;
}

and then:

{
  "css": [
    "~/App_Plugins/BackOfficeTheme/index.css"
  ],
  "bundleOptions": "None"
}

Giving this result:

Hope this helps!

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