Topology image transparency problem

In chapter [Router Exercise](https://github.com/mininet/openflow-tutorial/wiki/Router-Exercise), it seems that the image that shows the basic topology is trasparent something that causes the following problem when GitHub's dark theme is enabled:
![image](https://github.com/mininet/openflow-tutorial/assets/13571540/227868e0-f787-4a17-a601-092517b445da)
