To add some crackpot, from the themes I see in the game, I came to think that Light vs Dark is less about Good vs Evil, but more about selflessness vs selfishness. Dark themes constantly are about seeking power and freedom (freedom being a generally accepted good thing), while Light themes generally involve helping and protecting others, but also sacrifice for the "greater good".
This also means a "balance between light and dark" can manifest in multiple ways: High sacrifice to gain power and order, or helping others while remaining free.

This also can be applied to consider what Hydaelyn is about.. she is a manifestation of selflessness, and therefore is willing to sacrifice herself to protect her children, even if that would mean she loses the power to protect later (or even ceases to be), that falls in line with that Hydaelyn keeps getting weaker and weaker, and is now (and maybe for a longer time already) weak enough she has to pick specific people to protect that can in turn protect others, rather than intervening on a larger scale. Without a bit of selfishness, she won't gain any of the power she spent back unless someone directly gives it to her (like Minfillia did in 3.2, ironically by sacrifice).