Use different background color for PR view of newly added files?

When skimming through a PR it's often hard to distinguish between files that only have whitespace modifications (where nothing is highlighted), and completely new files. The tendency of many devs is simply not to bother reading through the code added in new files, as they're just looking for the "colored" diffs. 

I can see in the HTML that the former have the class "modified-in-monaco-diff-editor" in one of the containing divs, whereas the latter don't, so in principle it should be possible to override, say, the CSS background color, e.g. using a pale green for new files, but I'm not sure where/how I would do these customizations, other than via a browser extension.