docs: make package-slices.svg readable in dark mode

[gcomneno]

Fixes #840

The package slicing SVG used black strokes/fills on a transparent background, making arrows/text hard to see in dark mode. Use currentColor for black stroke/fill so the diagram follows the documentation theme.

[medubelko]

This renders the same in the preview:

https://canonical-ubuntu-documentation-library--1108.com.readthedocs.build/rockcraft/1108/explanation/chisel/

It might be easier to give the lines a neutral grey that works in all modes.

[gcomneno]

Good catch — I've pushed an update that replaces currentColor with consistent neutral greys for both strokes and labels in package-slices.svg, so it should render clearly in both light and dark themes. The preview should now reflect the change.

[medubelko]

Looks good, thanks!

[gcomneno]

Noticed that one slow test job (jammy, amd64, 3.10) is failing, while the rest of the checks are passing. Since this PR only adjusts an SVG in the docs, this might be unrelated. Could this be rerun to confirm whether it's a flake?

[asanvaq]

Hi @gcomneno! These are expected/persistent CI failures. This also applies to all the other PRs.

[gcomneno]

Thanks for clarifying — understood. I’ll treat those failures as persistent/unrelated and won’t take any further action on this PR (or any other PRs) unless something specific comes up.