The lineColor is not controlled by darkMode option provided by Mermaid

[packetmania]

Describe the bug In the dark mode on Macbook Pro, the lines are still with dark color, which makes them invisible.

To Reproduce Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior In the dark mode, the lines can go to light color automatically.

Screenshots If applicable, add screenshots to help explain your problem.

Code Sample If applicable, add the code sample or a link to the live editor.

Desktop (please complete the following information):

• OS: macOS
• Browser Chrome
• Version 91.0.4472.114

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context Add any other context about the problem here.

[DaweiX]

The same issue exists on Windows.