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I'm ussing Steam Beta Client and since the April 30 update, it doesn't recognize skins in ~/.local/share/Steam/skins/ folder.
I've tried Metro for Steam (http://metroforsteam.com/) and PixelVision (http://pulseh.deviantart.com/art/PixelVision-Skin-for-Steam-211426153)
I don't have a windows installation to try if this is a problem with both clients or if this is a problem only in linux.
System Info: http://pastebin.com/Uxq5ihHi
I have the same issue, though with my own custom skins, and the issue is now present in the main client in Linux. The issue is not present in Windows, though I don't know about OS X.
A future Steam beta client (not sure which one) will have a fix for this.
Still not fixed. Using the current beta, skins are not showing up in the settings dropdown.
Today's beta client update should fix this.
Confirmed fixed, thanks!