If I'm not mistaken, this is just because the CSS styling isn't that good, right? Take Slack (which uses Electron) as an example: it looks almost the same on Windows and Mac, but it looks good — because the styling (through CSS) is good.
Slack looks nice but feels quite broken. For example, its non native context menus are beautiful, but do not dismiss properly, do not support single-click selection, do not support spacebar to pick, do not support type-select...
When I use Slack I am constantly frustrated that things don't work the way my other Mac apps work.