Sadly, as of writing this, there is no official way of using skins on the new Steam UI right now. There are however some kind of CSS hacks via a third-party tool the community made (since the new steam client shares a framework for Desktop-Client, Web-Client and the SteamDeck itself). These hacks are limited in their functionality and wouldn't work at all for most skins out there (e.g. you can't edit/move bottom and top bars)
If Valve decides to bring back functionality for skins it might be in form of CSS rather than the old VGUI-stuff. Until then this project is dead.
A Windows 10 Style Steam Skin
Download
Preview
Steam Community Group
Wiki
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Download the skin, extract the zip file, and copy the folder to:
- Windows - C:\Program Files (x86)\Steam\skins
- Mac - /Users/<username>/Library/Application Support/Steam/Steam.AppBundle/Steam/Contents/MacOS/skins/
- Linux - ~/.steam/skins
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In Steam, go to Settings > Interface > and choose Threshold from the list of skins.
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Restart Steam and enjoy!
Note: Mac & Linux support is not complete