Made the theme switcher more accessible

[royeden]

Accessibility is important, even for demos :smile:

Keyboard fixes:

• Switched from a to button, as the a without an href wasn't keyboard focusable. This also allows you to switch themes back and forth using the space/enter keys.
• Added focus/blur previews and simplified into a previewStart/previewEnd functions.

Aria fixes:

• The role attribute should be "switch" to indicate the context of the action.
• Added aria-label to clarify what the button does (the screen reader will read the button as dark theme switch on/off).
• Added aria-checked to showcase the current state of the button.

Everything was based from https://www.radix-ui.com/docs/primitives/components/switch