This refactors popover list structure to use a flattened `ul > li`
structure along with aria-related additions, enabling screen reader
accessibility and announcing menu length and position (`# of n`).
Added ARIA roles:
- `role="menu"` to the parent `<ul>` element, indicating it is a widget
offering a list of choices.
- `role="menuitem"`, `role="menuitemcheckbox"`,
or`role="menuitemradio"` to child elements based on their function.
- `role="group"` to the theme switcher, identifying it as a container
for related menu items.
- `role="none"` to `<li>` elements, removing the implied `listitem`
role that conflicts with the parent menu structure.
Sayam Samal