Readme.md: State Diagram graphics can't be read with Dark Theme / Dark Mode on Github

[alos-source]

Description
In the README.md, the lines between the boxes of the class diagrams are barely visible when the GitHub appearance settings are in dark mode. This might also apply to other markdown viewers supporting a dark theme.

The reason is the linked PNG graphics:

• https://github.com/simatic-ax/statemachine/blob/main/docs/state.png
• https://github.com/simatic-ax/statemachine/blob/main/docs/guard.png
• ...

Those are be displayed like this:

To Reproduce
Steps to reproduce the behavior:

1. Set Github Appearance to "Dark Theme": https://github.com/settings/appearance
2. Have a look at the Readme: https://github.com/simatic-ax/statemachine and refresh page, if it has been open before.

Expected behavior
Graphics should work independant from the used theme. So black lines on transparent background should be avoided.