Dark Mode & Priority Enhancement

You recently fixed a bug in my React-based to-do app, and I’d like to continue working with you on two small but important upgrades.

- Dark mode — toggle button, whole app switches, and it should stay dark if I refresh the page (last time something like this reset on refresh, don't want that again).

- Priority — each todo can be Low / Medium / High. The color of the todo item changes depending on priority so I can see it at a glance. Doesn't need icons or anything fancy, just color.