I also like the aesthetics of OS X versions prior to 10.10. It is hard to find programs that still support older versions though. To make things easier I'm using the more modern macOS 10.14 but installed a few things to make it look more like 10.9.
macOSLucidaGrande (https://github.com/LumingYin/macOSLucidaGrande) can change the system font back to Lucida Grande like 10.9 used. One annoyance with this though is that the '*' character won't show up in password input fields.
The file and instructions in https://forums.macrumors.com/threads/mavericks-window-contro... change many of the UI elements to look like those in 10.9.
Menu Bar Tint (https://manytricks.com/menubartint/) can make the menubar look like the one in 10.9.
macOSLucidaGrande (https://github.com/LumingYin/macOSLucidaGrande) can change the system font back to Lucida Grande like 10.9 used. One annoyance with this though is that the '*' character won't show up in password input fields.