Add light and dark modes

[smarquez1]

Purpose

Implement intentional light and dark modes for the portfolio.

Scope

• Support system preference.
• Add an accessible manual toggle as progressive enhancement.
• Preserve contrast and focus visibility in both modes.

Acceptance Criteria

• The site has usable light and dark themes.
• The default works without JavaScript through system preference.
• Manual toggle is keyboard accessible.
• Theme choice does not hide or degrade content.