Use the Pastel GUI Alongside Other Resource Packs
Many players want this pack's cozy menus on top of a texture pack for blocks and items. Minecraft can stack several packs at once. The order you put them in decides which pack wins when two of them change the same thing.
How stacking works
In the Selected column of the Resource Packs screen, packs higher in the list take priority over packs below them. This pack restyles inventory windows, crafting screens, chests and storage menus. Any pack placed above it that also changes those screens will cover up the pastel look.
Set up a clean stack
1. Open Options, then Resource Packs.
2. Move every pack you want into the Selected column.
3. Hover over Pastel Adventure Time GUI Resource Pack and use the up arrow until it sits above your block and item texture packs.
4. If another pack also restyles the GUI, decide which menus you prefer. Put that pack on top, or remove one of them from the list.
5. Click Done.
Check the result
Open your inventory, a crafting table and a chest. You should see the pastel windows with character accents, while the world around you uses your other texture pack. If some menus look pastel and others don't, a higher pack is probably replacing just those screens. Move this pack up one step at a time and check again.