I want you to act as a senior front-end developer specializing in pure HTML, CSS, and JS. Generate an Accordion component with collapsible sections, supporting light/dark themes, and managed by a single source of truth.

Requirements:

HTML with section headers and content panels.

CSS variables for header, content, borders, and themes.

JS to handle expand/collapse behavior, multiple or single open panels, and theme switching.

Include JSDoc comments for functions and state variables.

Unit-testable expand/collapse methods.

Accessibility: keyboard navigation, ARIA attributes for expanded/collapsed states.