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:
Please share some love and up vote if you like this theme and make sure to check out my SDDM login theme that surely fits your new window decoration!
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.
Install 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. Comment bugs and anything else.