fix: correct description text color in overview pages

[houhoucoop]

Summary

Fixes #15169

Occurred changes and/or fixed issues

• On the Longhorn overview page, the description text has the same color as --body-bg, making it invisible in both light and dark themes.

Technical notes summary

• Remove the color from the CSS so that the description uses the var(--link) color

Areas or cases that should be tested

• istio page
• longhorn overview page
• neuvector page

Areas which could experience regressions

N/A

Screenshot/Video

Before

After

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
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• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
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• The PR has considered, and if applicable tested with, the three Global Roles Admin, Standard User and User Base

[rak-phillip]

LGTM - Good catch!

--secondary is one of those color variables that was used, but never defined in the original theme. We've run into issues with this in other places like inputs.