Add Pi-hole darker theme

[DL6ER]

By submitting this pull request, I confirm the following:

• 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, and have included unit tests where possible.
• 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)

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What does this PR aim to accomplish?:

This PR adds a third default theme, an even darker variant of the dark one. It also fixes an issues with the existing dark theme where links in box headers (like the Query Log's "show all") were completely invisible:

old:

fixed:

How does this PR accomplish the above?:

Add a new theme. Comparison between the existing dark (on the left) and the new darker (on the right):

What documentation changes (if any) are needed to support this PR?:

None, this new theme is an opt-in feature.

[pralor-bot]

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

https://discourse.pi-hole.net/t/new-unknown-0-status-showing-up/43694/28

[PromoFaux]

Darker is always better in my eyes :)

[metool6]

How do I add this to my pi?

[DL6ER]

The next update of Pi-hole will include it.

[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-v5-8-web-v5-5-and-core-v5-3-released/46237/1