fix: soften dark mode from pure black to dark gray

[rajnavakoti]

Bug

Dark mode uses pure black background which feels harsh and overly dark.

Expected Behavior

Dark mode should use softer dark tones for a more comfortable reading experience.

Acceptance Criteria

• Dark mode background changed from pure black to softer dark gray
• Dark mode surfaces use layered grays for depth
• Text contrast remains WCAG AA compliant (4.5:1 minimum)
• Cards and sections have subtle visual separation in dark mode

Out of Scope

• Light mode changes (unless needed for consistency)