theme: Fix icon theme switching changing the theme (#24849)

Closes #24846 

Release Notes:

- Fixed an issue where changing the icon theme would change the theme.

crates/theme/src/settings.rs

@@ -474,7 +474,7 @@ impl ThemeSettingsContent {
 
             *icon_theme_to_update = icon_theme_name.to_string();
         } else {
-            self.theme = Some(ThemeSelection::Static(icon_theme_name.to_string()));
+            self.icon_theme = Some(IconThemeSelection::Static(icon_theme_name.to_string()));
         }
     }