There are 3 repositories under steam-skin topic.
A skin to make Steam look more like a native GNOME app
A simple Gtk wrapper for Adwaita-for-Steam
🕹️ Soothing pastel theme for Steam (WIP)
A modern and darker UI for steam (this skins is known as the Metro Skin)
Steam skin which tries to look like a native app for elementary OS and macOS
A wiki on how to skin steam
A sleek and elegant theme that brings a darker, more immersive experience to your Steam client.