Always show a warning when theme is unknown

src/features.rs

@@ -57,10 +57,7 @@ pub fn list_languages(assets: &HighlightingAssets, term_width: usize) {
 }
 
 pub fn print_files(assets: &HighlightingAssets, config: &Config) -> Result<bool> {
-    let theme = assets.get_theme(match config.theme {
-        Some(ref theme_name) => theme_name,
-        None => "Default",
-    })?;
+    let theme = assets.get_theme(&config.theme);
 
     let mut output_type = OutputType::from_mode(config.paging_mode);
     let handle = output_type.handle()?;