How to install the DF11 base skins on FM19
Download the .rar archive and use a tool such as 7-zip for Windows or The Unarchiver for MacOSX to extract the folders.

Move the 3 extracted folders to:
\Documents\Sports Interactive\Football Manager 2019\skins
Create folder "skins" if it doesn't exist already.

Start the game and go to Preferences screen, and then to the Interface tab.
You should see "DF11 base skin 2019", "DF11 Base skin 2019 - Dark" and "DF11 Base skin 2019 - Light" as options in the skin drop down of the Overview box.
Use the Clear Cache button, then hit the Confirm button.