[Solved] Remove gradient background in 'private' Newtab

Using Firefox Beta

Hi, I'm trying to remove the gradient in 'Private mode' new-tab page, I'm trying the next code but without success.

@-moz-document url-prefix(about:privatebrowsing) {
html.private {
        background-color: transparent !important;
        background-image: none !important;
        background: linear-gradient(45deg, transparent, transparent) !important;
    }
}

I also tried:

@media -moz-pref("browser.privatebrowsing.felt-privacy-v1") {
  @media not (prefers-contrast) {
    html.private {
      background: none !important;
    }
  }
}

But with all the gradient don't disappear.