feat(theme): implement FileNameColours.icon for Theme

Signed-off-by: Sandro-Alessio Gierens <sandro@gierens.de>

src/theme/mod.rs

@@ -372,6 +372,10 @@ impl FileNameColours for Theme {
     fn executable_file(&self)     -> Style { self.ui.filekinds.executable }
     fn mount_point(&self)         -> Style { self.ui.filekinds.mount_point }
 
+    fn icon(&self) -> Option<Style> {
+        self.ui.icon.map_or(None, |icon| Some(icon))
+    }
+
     fn colour_file(&self, file: &File<'_>) -> Style {
         self.exts
             .get_style(file, self)