I developed this app years ago when Steam allowed changing skins with relative ease, although it still works today, the skins have problems working and I cannot guarantee that all the skins work.
Below I detail how it works:
Selection of skins
First of all, the app offers you a list of skins available to change Steam.
Downloading and installing skins
In the selected skin, just click on the Download button for the skin to be downloaded and installed automatically in Steam.
How it looks on Steam
This is how the new selected appearance appears on Steam.