The fundamental nature of being is good. At first glance, this doesn't seem obviously true. There are a lot of bad things in the world: suffering, drudgery, malevolence. But to me this abstract point is the most important idea which ties everything together. It is the foundation of ethics and how to think about living well.
Part of this idea comes from Christianity: God's goodness runs in everything from exalted to the everyday. Jesus Christ is not just a perfect God but also a messy human being. Jesus is both God and Man, eternal and creaturely, high and low, poetry and mud, perfection and blood and guts. Instead of only worshipping perfection, and seeing all outside of that as bad, goodness is in everything. The idea is in the Jewish tradition and the Old Testament too: God made the world and it is all good. The poetry and the mud are both good. The purity and the squalour are good. Hence we should love all of it. Everything is in fact worthy of love.
But there are levels of goodness. Some things participate more in the pure, eternal form of goodness than others. Though human beings will always be messy, we can make ourselves closer to goodness. This idea comes from Plato: we can educate ourselves to become more noble. So we don't give up on the idea of perfection. Genuinlely pure and perfect goodness really is wonderful. And we should try to get closer to it.
Plato thought the more true goodness is achieved, the more is has a simplicity and a clarity to it. Nobility and wisdom is simple, confident, uncomplicated, like a monk who is calm, perhaps playful, and who speaks simply. (Note: the reverse is not true: if something is simple and confident that doesn't mean it is noble.) All the different kind of "badness" are forms of confusion, like messing up a nice garden. The messy garden is never totally destroyed; it still has some goodness to it - there is still life and flowers - a but it can be improved and made more beautiful by being better ordered.
This is also the idea behind Darth Vader in Star Wars. Darth Vader is pretty messed up but there is still goodness in him. There is always goodness because being itself is good, even if it is only seen via a faint signal, muffled beneath layers and layers of confused habits, defensiveness and internal disorder (in the case of Darth Vader, actual armor and a mask). When someone is more ordered in the right way, goodness shines out like a bright light - in playfulness, sincerity, joy, kindness, calm, strength and wisdom. When someone is more messed up, that light is hidden. Darkness is not a thing in itself, badness is not a force; darkness is just an absence of light. Badness is just confused being.
One central lesson of all this for me is that your life should be oriented around something good, not something you dislike. We should always be talking about what we are for, not what we are against. This idea comes from Nietzsche: if you orient your life's efforts around something that pisses you off (like protestors focusing on what they hate), you will be poisoned by this. You are not tapping into anything good; you are degrading and messing yourself up by just hating on the disorder. The best way to get rid of shadows is by bringing in the light.