feat: theme-level blocks.css takes priority over platform asset

Co-Authored-By: Claude Code <noreply@anthropic.com>

tests/test_theme_registry.py

@@ -91,3 +91,16 @@ def test_env_for_degraded_branch_not_cached_and_single_path(tmp_path, monkeypatc
         assert 'industrial' not in theme_pro._ENVS
     finally:
         core.THEMES = core.discover_themes()
+
+def test_theme_blocks_css_overrides_platform(tmp_path):
+    from app import builder
+    dist = tmp_path / 'assets'; dist.mkdir()
+    theme_dir = tmp_path / 'theme'; theme_dir.mkdir()
+    (theme_dir / 'blocks.css').write_text('/* theme version */', encoding='utf-8')
+    builder.copy_blocks_css(theme_dir, dist)
+    assert (dist / 'blocks.css').read_text(encoding='utf-8') == '/* theme version */'
+    # 无主题版时回落平台版
+    dist2 = tmp_path / 'assets2'; dist2.mkdir()
+    builder.copy_blocks_css(tmp_path / 'nonexistent', dist2)
+    assert (dist2 / 'blocks.css').is_file()
+    assert (dist2 / 'blocks.css').read_text(encoding='utf-8') != '/* theme version */'