Warn instead of failing for themes

src/librustdoc/config.rs

@@ -373,10 +373,9 @@ impl Options {
                 }
                 let (success, ret) = theme::test_theme_against(&theme_file, &paths, &diag);
                 if !success || !ret.is_empty() {
-                    diag.struct_err(&format!("invalid theme: \"{}\"", theme_s))
+                    diag.struct_warn(&format!("invalid theme: \"{}\"", theme_s))
                         .help("check what's wrong with the --theme-checker option")
                         .emit();
-                    return Err(1);
                 }
                 themes.push(theme_file);
             }

src/librustdoc/theme.rs

@@ -273,6 +273,7 @@ pub fn test_theme_against<P: AsRef<Path>>(
     diag: &Handler,
 ) -> (bool, Vec<String>) {
     let data = try_something!(fs::read(f), diag, (false, vec![]));
+
     let paths = load_css_paths(&data);
     let mut ret = vec![];
     get_differences(against, &paths, &mut ret);