Skins for KM 1.5.x can mainly be found here:
And of course in this forum here, but you found that already
To apply them open your KM folder in Explorer (not the browser, but the file manager).
It contains a folder "skins", in it are all skins, each has their own folder.
Copy the new one there too, then start KM and click Edit/Preferences/GUI to choose one of the skins. Needs browser restart.
If you don't know how to find the skins folder, you could alternatively open KM and click Edit/Configuration/Macros, then go up one level.