Checkbox and Radio button styling

[nevf]

@finnhvman I've just realized that both checkbox and radio buttons use the user agent stylesheet for their unchecked/unset state which doesn't seem right to me. I would have thought they'd use --matter-helper-theme which is what the checked and indeterminate states use. ie.

.matter-checkbox > input[type="checkbox" i] {
    border-color: rgb(var(--matter-helper-theme));
}