Tested with Plasma 5.x
This is a flatter version of my theme Breeze OSX for the default KDE desktop environment. If this is too flat for you check out Breeze OSX:
Based on the existing theme Evolvere Light Pure Circle by Frank Souza:
The default border width (config settings set to "normal") is 8px. This is by intention. 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.
Preferably installed via "System Settings > Window Decorations > Get New Decorations".
You can manually download the compressed directory, uncompress it and put it into ~/.local/share/aurorae/themes.
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, BorderRight and TitleEdgeBottom.
If you would like to use this theme without borders set the config settings to "tiny" and TitleEdgeBottom in Breeze-OSXrc to 0.
Either of this might throw off paddings which would need additional adjustments in the same file.