[docs]: Diagrams are hard to read in dark mode

[tonejito]

Version

Documentation website

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Describe the bug

When reading the documentation page in dark mode, the diagrams are hard to read

• https://pulpproject.org/pulp_deb/docs/user/guides/sync/

• https://pulpproject.org/pulp_deb/docs/user/guides/sync.svg

• 

• https://github.com/pulp/pulp_deb/blob/main/docs/user/guides/sync.md

• https://github.com/pulp/pulp_deb/blob/main/docs/user/guides/sync.svg

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To Reproduce

Open the docs web site:

• https://pulpproject.org/pulp_deb/docs/user/guides/sync/

Look at the diagram in light and dark mode

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Expected behavior

The diagram images have gray arrows that can be distinguished in either white or black background.

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Additional context

Setting the black arrows to a ticker line and 50% gray might solve this issue.

This issue affects many diagrams, so I suggest that the diagram patch is applied to all diagrams.