Reduce border size of line charts

[yubiuser]

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)
• I have Signed Off all commits. (git commit --signoff)

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What does this PR aim to accomplish?:
It reduced the border width of the line charts (both at the dashboard as well as the long term graph). This is best seen at time points where no data is present. It looks like the y-Axis (zero line) is bold black, but indeed it is only the border of the graph lines sitting at zero.

Before:

After:

[rdwebdesign]

Did you test it with the light theme?

I think it's better, but the line is still visible.

[yubiuser]

Yes, the border is still visible, but a lot less intrusive than before

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-16-web-v5-13-and-core-v5-11-1-released/56384/1