AdwSteamGtk

Make Steam look like it belongs on your computer. This app installs and updates the Adwaita-for-Steam skin for you. It's as simple as this:

• Configure the skin to your liking
• Click the “Apply” button
• Restart Steam
  On startup, the app will automatically download any updates to the skin and display a notification. Apply the skin again and restart Steam to take the changes into use.

In order to get rounded corners on Steam, you will need to use the “Rounded Window Corners” GNOME extension.