Improve colors of graph adjacency matrix

[JMCanning78]

Simply setting the bg or background attribute of buttons and text entry widgets does not seem to affect the displayed background color in the graph adjaceny matrix. I think it would look better if the vertex's color extended across a complete row. The initial implementation only shows the colors along the (static) diagonal:

One way to do this would be to make a custom widget from a Label or Frame class that has bindings to handle clicks and maybe keyboard input.

[JMCanning78]

For the weighted graph, the adjacency matrix cells are editable text entry areas. Those are showing the background color, although they are surrounded by frame of the default background.

[JMCanning78]

This appears to be a problem specific to macOS. Here's how the adjacency matrix appears for Graph.py on Windows:

There appears to be Python library that fixes this issue:
https://pypi.org/project/tkmacosx/