feat(ui): implement dark/light theme switch

tests/test_web_server.py

@@ -87,7 +87,7 @@ def test_readonly_routes_and_static():
             assert "text/html" in headers["Content-Type"]
             assert b"<html" in payload.lower()
 
-            for asset in ("/static/style.css", "/static/app.js", "/static/vendor/echarts.min.js"):
+            for asset in ("/static/style.css", "/static/app.js", "/static/theme.js", "/static/vendor/echarts.min.js"):
                 status, asset_headers, asset_payload = running.request(asset)
                 assert status == 200, asset
                 assert asset_payload, asset