Dark Mode GUI is a resource pack, so you switch it on from the game's resource pack screen rather than from a mods menu.
Enable the pack
1. From the main menu or the pause menu, open Options and then Resource Packs.
2. Find Dark Mode GUI in the Available list on the left. If it is missing, click Open Pack Folder and confirm the pack file is inside the resourcepacks folder. Then reopen the screen.
3. Click the arrow on the pack to move it into the Selected list on the right.
4.
Walk through the menus
Check each screen the pack is meant to restyle:
- Inventory: Press your inventory key. The bright grey background should now be dark. In Creative mode, scroll through a few creative tabs as well.
- Crafting Table: Place one and open it. The grid area should match the dark style.
- Chest and Furnace: Open both. These are the menus that most often glare at night, so they should look noticeably darker.
- Enchanting Table: Open it to see the cleaner, minimalist layout.
- Advancements: Open the advancements screen and look at a tab such as The End. The interface around the progress icons should be dark.
If one of these screens still looks bright, another resource pack is probably overriding it. Move Dark Mode GUI higher in the Selected list.