Added base theme for defaults

src/theme.rs

@@ -31,6 +31,10 @@ pub fn discover_themes() -> Result<Vec<String>> {
             let Some(name) = path.file_name().and_then(|name| name.to_str()) else {
                 continue;
             };
+            // "base" is a reserved layer name, not a selectable theme.
+            if name == "base" {
+                continue;
+            }
             themes.push(name.to_string());
         }
     }