onboarding: Expand power of theme selector (#35421)

Closes #ISSUE

The behavior of the theme selector is documented above the function,
copied here for reference:
```rust
/// separates theme "mode" ("dark" | "light" | "system") into two separate states
/// - appearance = "dark" | "light"
/// - "system" true/false
/// when system selected:
///  - toggling between light and dark does not change theme.mode, just which variant will be changed
/// when system not selected:
///  - toggling between light and dark does change theme.mode
/// selecting a theme preview will always change theme.["light" | "dark"] to the selected theme,
///
/// this allows for selecting a dark and light theme option regardless of whether the mode is set to system or not
/// it does not support setting theme to a static value
```

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/ui/src/components.rs

@@ -34,7 +34,6 @@ mod stack;
 mod sticky_items;
 mod tab;
 mod tab_bar;
-mod theme_preview;
 mod toggle;
 mod tooltip;
 
@@ -77,7 +76,6 @@ pub use stack::*;
 pub use sticky_items::*;
 pub use tab::*;
 pub use tab_bar::*;
-pub use theme_preview::*;
 pub use toggle::*;
 pub use tooltip::*;