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/theme/src/settings.rs

@@ -438,7 +438,7 @@ fn default_font_fallbacks() -> Option<FontFallbacks> {
 
 impl ThemeSettingsContent {
     /// Sets the theme for the given appearance to the theme with the specified name.
-    pub fn set_theme(&mut self, theme_name: String, appearance: Appearance) {
+    pub fn set_theme(&mut self, theme_name: impl Into<Arc<str>>, appearance: Appearance) {
         if let Some(selection) = self.theme.as_mut() {
             let theme_to_update = match selection {
                 ThemeSelection::Static(theme) => theme,