Some default theming for form-item--checkboxes.
We also have the theming for a switcher variation of the checkbox included.
Using the switcher variation
Single checkboxes can be used as switchers by setting the variation variable as 'switcher':
$form['switcher'] = [ '#title' => $this->t('Switcher variation'), '#type' => 'checkbox', '#variation' => 'switcher', ];
Feel free to use this variable to come up with different variations.