Check Steam Millenium

https://developer.valvesoftware.com/wiki/Steam_Skins

There’s also SFP (formerly SteamFriendPatcher) & CSSLoader but most people just use Millenium.

Originally posted by Tommy:

hello, i am an frontend developer and i wanted to know if i could somehow access my steam clients css code to change it to or use an app for changing the UI

does anyone know if you can access and change the files or an app suggestion?

All these (third-party) "injection things" and "loader" from external sites are completely unnecessary.

Editor: e.g. Notepad++
Image editing program: e.g. Adobe Photoshop or whatever you prefer to edit/replace graphics/textures

Client GUI files like header, footer, menus:
x:\Steam\resource - *.styles, *.layout, *.menu

Library
x:\Steam\steamui\css\library.css

Everything else like web interface, including profiles, store pages, forum pages whatever: x:\Steam\resource\webkit.css

Add or modify your own things. However, since Steam forces all modified files to reset at startup, updates must be blocked.

Create a "steam.cfg" in your installation folder.

Add:
Save and close the file

If you want to make updates again, the cfg must be renamed or moved temporarily. At that moment, Steam will update again during a new launch. Of course, you should back up your modifications beforehand, otherwise you'll be annoyed.

Hints:
Launch steam with "steam.exe -dev" shortcut to unlock the Code inspector (F12)
(F5) to reload the Client

There was also an additional feature that allowed you to view the CSS hierarchy, but unfortunately it was removed later. Valve often changes the CSS code, renames classes, files or move them to different places. You always have to make sure you're up to date.

Notepad++ VDF Plugins (optional)
https://developer.valvesoftware.com/wiki/Notepad++_VDF_Language_Plugins
https://github.com/Mrfunreal/VDF-Lanugages-for-NotepadPlusPlus