Confluence 4.2.4 shows grey borders around code blocks

Confluence 3.5's code macro used to contain a white background. Upgrading to 4.2.4 has revealed that the code macro now contains an ugly grey thick border.

I tried applying a global style override but it appears too early and gets overridden and there's no way to remove this border.

div.code.panel {
  background-color: inherit;
}