I put these items that go in slots that are for attack def w/e....some are lit up while rest are dark..does that mean the dark ones aren't active?
Correct. Gotta have enough mana to equip what you want.
Also look out for joint items, they're really powerful items but require tons of mana to use(if you equip with joints to it, if not, they're just like normal items), I'll try to explain a bit of the basics to help you understand. Say we equipped two 2atk "joint" items together, both costing 2mana, when we do, it should give us 4 atk for 4 mana normally, however, since they're joint, each item in the chain is the sum of the whole chain, both in stats they give and mana cost, so each piece would give 4atk for 4 mana and since we have two in the chain that means the whole chain gives 8 atk for 8 mana.