Minecraft loads resource packs in layers. A pack higher in the Selected list overrides the ones below it. If you run a texture pack alongside Dark Mode GUI, the other pack may bring back bright menu textures.
Reorder your packs
1. Open Options and then Resource Packs.
2. In the Selected column, hover over Dark Mode GUI and use the up arrow to move it to the top of the list.
3. Click Done and let the game reload.
4. Open a Chest, Furnace and Crafting Table to confirm they are dark again.
With Dark Mode GUI on top, its menu textures win. Your other pack can still handle blocks, items and anything else Dark Mode GUI does not touch.
Track down a stubborn conflict
If one menu still looks wrong after reordering:
- Temporarily move every other pack back to Available, leaving only Dark Mode GUI selected.
- Check the problem menu. If it is now dark, add your other packs back one at a time until the bright screen returns. That shows you which pack is responsible.
- Keep that pack below Dark Mode GUI. If you prefer that pack's menus, leave Dark Mode GUI off for that screen's sake.
On a server that sends its own resource pack, the server pack can override your menu textures too. The fix is the same idea: the topmost pack decides what you see.