Fix hyperlink font color in Dark mode

[ruokun-niu]

Description

This pull request improves the accessibility and clarity of hyperlink styles in the documentation's dark mode. It updates the styling for plain prose links to ensure they have sufficient contrast and that visited links are visually distinct from unvisited ones.

Accessibility and link color improvements:

• Updated .td-content a:not([class]) to use a bright cyan ($accent-cyan) for unvisited links and a soft purple ($accent-purple) for visited links, ensuring AAA contrast and clear differentiation between link states. Hover and focus states are lightened for both colors.

Type of change

• This pull request fixes a bug in Drasi and has an approved issue (issue link required).
• This pull request adds or changes features of Drasi and has an approved issue (issue link required).
• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and doesn't change the functionality of Drasi (issue link optional).

Fixes: #244

[Copilot]

Pull request overview

Updates the documentation theme’s dark mode styling to improve readability and accessibility of in-content (“prose”) hyperlinks by increasing contrast and clearly differentiating visited vs. unvisited states.

Changes:

• Add a dark-mode rule for .td-content a:not([class]) to use $accent-cyan for unvisited links and $accent-purple for visited links.
• Add hover/focus variants for both unvisited and visited states using lighten(...).

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