metamask-extension/ui/pages/create-account/create-account.scss

.new-account-wrapper {
  background-color: var(--color-background-default);

  @media screen and (min-width: $break-large) {
    width: 85vw;
  }

  @media screen and (min-width: 768px) {
    width: 80vw;
  }

  @media screen and (min-width: 1280px) {
    width: 62vw;
  }
}