The Utopia Extension adds dark screens for other mods on top of Mandala's GUI – Dark mode. It is meant to run alongside that main pack. If you enable only the extension, vanilla menus such as your inventory, chests and furnaces will not get the dark look from the main pack.
Enabling both packs
1. Make sure both pack files are in your resourcepacks folder. The quickest way to open it is Options → Resource Packs → Open Pack Folder.
2. Return to the Resource Packs screen. Both packs should appear in the Available list on the left.
3. Click the arrow on Mandala's GUI – Dark mode to move it to the Selected list.
4. Do the same for the Utopia Extension.
5.
Getting the order right
In the Selected list, packs nearer the top override packs below them. Keep the Utopia Extension above the main Dark mode pack, so its mod-specific screens are applied on top of the base dark style. Use the small up/down arrows that appear when you hover a selected pack to reorder it.
Quick check
1. Open your normal inventory. It should be dark, which comes from the main pack.
2. Open a menu from one of your mods, such as a backpack or a machine. It should also be dark, which comes from the extension.
If only the first check passes, recheck the order or confirm the extension is actually in the Selected list.