Not every screen will turn dark after you enable the extension. This is usually by design, not a bug.
Screens the pack leaves alone on purpose
The extension repaints menus that use the vanilla style: the plain light-grey panels many mods borrow from Minecraft. If a mod already ships its own custom artwork for a menu, the extension leaves that artwork untouched.
That means a menu with a unique themed design will keep its original look. Only the standard grey-style menus get converted.
Working out what is happening
When a menu looks wrong, go through these checks:
1. Is it a custom design? If the screen has distinctive art rather than the plain grey panel, the extension is intentionally skipping it.
2. Is the mod covered? The pack supports over 110 mods, but not every mod in existence. A plain grey menu from an unsupported mod will stay bright.
3. Are both packs active? Go to Options → Resource Packs and confirm that both the main Dark mode pack and the extension are in the Selected list.
4. Is something overriding it? Another resource pack placed above the extension can replace the same menu textures. Move the extension higher and test again.
After making changes
Click Done on the Resource Packs screen so the game reloads. Then reopen the menu in question. A half-dark screen often comes from another pack replacing only part of the textures, so ordering is the first thing to try.