• SteamCustomizer[steamcustomizer.com]
  
• SteamSkins[steamskins.org]
  
• Skins[stskins.com]
  
• MetroforSteam[metroforsteam.com]
  
• BananaGaming[gamebanana.com]

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀First of all you need think about the skin you want or the theme.
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀In this case I will use my favorite skin MetroUI[metroforsteam.com]

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On the page in right corner will see a button.
Press on "DOWNLOAD[github.com]"

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When you download the file go on your
"My computer" and un-zip the folder

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Coppy the "MetroUI folder" to :
C:\Program Files (x86)\Steam\skins

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Open Steam > settings > interface "Change the default to metro ui skin"

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Press OK and restart the steam . Enjoy your new theme