Nostalgia

From Valve Developer Community

The green and grey skins, available here (http://files.filefront.com/Nostalgia/;6581451;;/fileinfo.html), make Steam look and feel like the older versions of Steam. Run the reg files and restart Steam to set the skin. The reg files will also set HL1's skin to use the corresponding skin. Be sure to install them to ..\Steam\skins.