Conditional Colorization based on Dark Mode State

### What is the problem or limitation you are having?

Some platforms the UI dark/light can autochange based on something like time, so we need to probably implement something similar to a "color set" specifying different colors to display under dark/light mode differently, and have it automatically switch using native APIs.

### Describe the solution you'd like

See above

### Describe alternatives you've considered

Using a handler for users to change all colors manually; if there's a huge bunch of colors, maybe won't work so well.

### Additional context

https://github.com/beeware/toga/issues/2841#issuecomment-3190224719

Start by reading the issue discussion and the linked Toga issue comment, then investigate how the target platforms expose native dark/light mode changes. Define the color-set behavior and switching scope before implementation; done means colors automatically follow the platform mode without requiring users to update them manually.

Written by the indexing model from the issue text.