Add Deepness theme stylesheet (css/deepness.css)

[TheWitness]

Summary

Adds a css/deepness.css stylesheet so this plugin's elements are tuned for the
new Deepness dark-emerald theme (proposed in Cacti/cacti#8202).

It mirrors the plugin's existing dark.css (low-opacity severity tints with
light text so the dark page shows through) and aligns it with the Deepness
design intent:

• Success / "OK" / "Up" / "restored" colours shift to the Deepness emerald accent.
• Dark surface colours are aligned to the Deepness panel palette.
• Error / warning / notice / acknowledge colours are kept (already dark-appropriate).

No behaviour change: Cacti only loads css/<theme>.css when that theme is the
selected theme, so this file is inert unless the Deepness theme is active.

[Copilot]

Copilot review overview

🟡 Changes recommended

The stylesheet does not implement the advertised Deepness-specific palette and omits the required changelog entry.

Review effort: Balanced
Findings: 1 · 2

Open (3)

• Add Deepness-specific emerald status color adjustments · New
• Document Deepness theme support in the changelog · New
• Rename copied dark theme comment to Deepness · New

What changed in this PR

Adds Deepness theme support for the Syslog severity legend.

Changes:

• Adds a dark-surface legend stylesheet for the Deepness theme.
• Defines severity-specific colors and layout.

File	Description
css/​deepness.css	Adds the Deepness-specific Syslog legend palette.

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