Fix CSS custom properties

[richardwestenra]

Description

Fixes a couple of regressions introduced in #301

Development notes

1. Fix invalid syntax in gradient colour - I had left in a , 1 when converting from rgba to var.
2. Invert filter-indicator-off colours - I'd gotten these confused because the light colours were suppose to be used on the dark themes and vice versa

QA notes

Fixes node-list tag icon colours on inactive tags when another tag is active

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added new entries to the RELEASE.md file
• Added tests to cover my changes

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