## ADDED Requirements

### Requirement: Header provides icon-based theme switcher
The system SHALL display a theme switcher in the top-right header area with icon controls for system, light, dark, and high-contrast modes.

#### Scenario: Theme options visible as icons
- **WHEN** a user views the header
- **THEN** all four theme options are represented by distinct icons
- **AND** each option is keyboard-accessible and screen-reader labeled

#### Scenario: Theme selection applies immediately
- **WHEN** a user selects a theme option
- **THEN** the page updates visual theme without full page reload
- **AND** selected option has a visible active state
