Design: Redefine docs color tokens and remove generic purple/cyan gradients

[hdcodedev]

Summary

Redefine docs color system and accents to avoid generic purple/cyan gradients and establish a clear brand direction.

Why

Current palette and gradients feel template-like and conflict with the skill guideline to avoid clichéd color schemes.

Scope

• Select one strong visual direction for color.
• Update root design tokens (primary/secondary/accent and semantic mappings).
• Update gradient usages in headings, active nav states, buttons, and chips.
• Keep contrast accessible in both dark and light themes.

Acceptance Criteria

• New palette is applied through CSS variables (not ad-hoc hardcoded colors).
• Existing purple-heavy gradients are replaced.
• Navigation/button/callout styles reflect the new direction.
• Contrast checks pass for interactive text and key UI states.

[hdcodedev]

Closed — addressed or superseded by modernization work in #44.