Fix the GUI Still Looking Like Vanilla After Installing the Pack
Quick checks
If your containers and inventory still have the standard gray frames, work through these steps in order.
1. Make sure the pack is selected. Open Options, then Resource Packs. The Unstable SMP GUI Pack must be in the Selected column on the right. A pack sitting under Available is installed but not active.
2. Check the load order. If another pack higher in the Selected list also changes the interface, it will cover this one. Move the Unstable SMP GUI Pack to the top.
3. Reload resources. Press F3 + T in-game to reload all resource packs without restarting.
4. Make sure the file is in the right place. Click Open Pack Folder in the Resource Packs menu. The pack should sit directly inside resourcepacks and not in a subfolder inside another folder.
On a server
Some servers send their own resource pack when you join, and that pack may restyle the interface too. If the GUI looks themed in singleplayer but not on a particular server, the server's pack is probably taking priority. Test in a singleplayer world to confirm that the Unstable SMP GUI Pack itself is working.
What should change
Keep in mind that the pack changes only menus, containers and inventory screens. Grass, ores, tools and other blocks or items will look the same as before. That is expected and does not mean the pack has failed.