Dark mode

Light, dark or the visitor's system setting, with no code per block.

Steps

1
Pick a mode
system follows the operating system, light and dark pin it.
```
<Theme theme="brand" mode="system">…</Theme>
```
2
Offer a toggle
The navbar's theme toggle sends kh:theme-toggle. Theme handles it; a later change of the system setting takes over again.
```
// navbar-simple props
{ themeToggle: true }
```
3
Remember the choice
Nothing is stored for you. If you want the choice to survive a reload, save it and pass it as mode.
```
const saved = localStorage.getItem('mode') ?? 'system';
<Theme mode={saved}>…</Theme>
```