The file I use to override these styles:
app/design/frontend/Vendor/ChildTheme/web/css/source/_module.less as per M2 documentation.
I tracked that it applies my modified styles, but then goes and puts the original on top, overriding my customisation.
Simple solution might be setting border: 0; but that's not what I want to achieve here. I need that rule removed.
To track it I added properties content: "Theme" and content: "Vendor" indicators. As you can see, the one from theme has border rule removed, but it gets overridden by rules from vendor.