Tested with Plasma 5.x
This is a high contrast version of my theme Flat OSX for the KDE desktop environment. If this is too much contrast for you check out Breeze OSX. The default border width (config settings set to "normal") is 8px. This is intentional. It makes for a consistent design throughout any application. All buttons got subtle indicators for their actions just like the recent OSX/macOS versions do, only a little less dominant/irritating.
Go to System Settings > Application Style > Window Decorations > Get New Decorations... and search for Flat
Extract to (change download path if necessary):
$ tar -xzvf ~/Downloads/Flat-OSX.tar.gz -C ~/.local/share/aurorae/themes
This will extract all necessary files to its location. After that go to the system configurations mentioned and just select the new theme.
If you would like to lower the border value you can easily change the [layout] section of Flat-OSXrc namely the values of BorderBottom, BorderLeft and BorderRight. If you would like to use this theme without borders set the config settings to "tiny".
This project is licensed under the CC-BY-NC-SA - see the LICENSE file for details
In the past years I have spent quite some hours on open source projects. If you are the type of person who digs attention to detail, know how much work is involved in it and/or simply likes to support makers with a coffee or a beer I would greatly appreciate your donation on my PayPayl account.