Hi Johan,
Yes, this setting is new in NT10, and indeed so is that file.
It's an important (system wide) setting because it means the border is included in width and height settings. This makes layout _much_ easier because you don't have to keep adjusting for the border width everywhere.
It may mean some tweaks to existing custom styles - yes, but the tweaks are simple enough, and make the whole theming process much easier.
yes, you can override it in your custom.css file, but I _really_ don't recommend that.
*, *::after, *::before {
box-sizing: whatever-you-want-it-to-be;
}
cheers
Bruce