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

Version

Documentation website

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

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

Expected behavior

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

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.

Start with docs/user/guides/sync.svg and its reference in docs/user/guides/sync.md, then compare the diagram on the documentation site in light and dark mode. Identify the other diagrams affected by the same styling and update them consistently. Done means the arrows are distinguishable against both backgrounds across the documentation site.