Add CSS variables for missing theme tokens causing hardcoded colors

[Wilfred007]

What

Several components use hardcoded color values (rgba, hex) instead of CSS variables. This breaks dark/light theme switching for those components.

Why

Theme toggle doesn't affect all components. Some elements remain the same color regardless of theme.

Scope

In scope: Audit all components for hardcoded colors, replace with CSS variables, complete variable set
Out of scope: Theme editor

Acceptance Criteria

• Zero hardcoded color values in components
• All colors use CSS variables
• Both themes render correctly
• No visual regressions

Technical Context

• frontend/src/index.css — CSS variables
• EmployeePortal.tsx has multiple hardcoded rgba values

[waterWang]

Submitted PR #574 to fix this issue. All hardcoded color values replaced with theme-aware CSS variables.