Use platform based path concatenation

src/bin/bat/main.rs

@@ -207,11 +207,11 @@ pub fn list_themes(cfg: &Config) -> Result<()> {
     }
     writeln!(
         stdout,
-        "Further themes can be installed to '{}/themes', \
+        "Further themes can be installed to '{}', \
         and are added to the cache with `bat cache --build`. \
         For more information, see:\n\n  \
         https://github.com/sharkdp/bat#adding-new-themes",
-        config_file().to_string_lossy()
+        config_file().join("themes").to_string_lossy()
     )?;
 
     Ok(())