Copy and replace the files in the Gfx folder of your ABL3 v3.2 installation. It’s a good idea to backup the original contents of the Gfx folder if you want to switch back to the default skin.
Default path to the ABL3 v3.2.2 installation folder is:
MS Windows: C:\Users\\Documents\AudioRealism\ABL3\
Apple macOS: System/Library/Application Support/AudioRealism/ABL3/