file_icons: Fall back to the default icon theme for icons (#23196)

This PR updates the various `FileIcons` methods to fall back to the
default icon theme if the active icon theme does not have the desired
icon.

Release Notes:

- N/A

crates/theme/src/registry.rs

@@ -12,7 +12,7 @@ use util::ResultExt;
 
 use crate::{
     read_user_theme, refine_theme_family, Appearance, IconTheme, Theme, ThemeFamily,
-    ThemeFamilyContent,
+    ThemeFamilyContent, DEFAULT_ICON_THEME_ID,
 };
 
 /// The metadata for a theme.
@@ -206,6 +206,11 @@ impl ThemeRegistry {
         Ok(())
     }
 
+    /// Returns the default icon theme.
+    pub fn default_icon_theme(&self) -> Result<Arc<IconTheme>> {
+        self.get_icon_theme(DEFAULT_ICON_THEME_ID)
+    }
+
     /// Returns the icon theme with the specified name.
     pub fn get_icon_theme(&self, name: &str) -> Result<Arc<IconTheme>> {
         self.state