Fix default UI theme ID in settings.

src/settings.cpp

@@ -27,7 +27,7 @@ math::Vec windowSize = math::Vec(1024, 720);
 math::Vec windowPos = math::Vec(NAN, NAN);
 bool invertZoom = false;
 float pixelRatio = 0.0;
-std::string uiTheme = "default";
+std::string uiTheme = "dark";
 float cableOpacity = 0.5;
 float cableTension = 1.0;
 float rackBrightness = 1.0;