$fields[] = new OOUI\FieldLayout(
				new OOUI\CheckboxInputWidget( [
					'name' => 'enableBeta',
					'selected' => $context->isBetaGroupMember(),
					'inputId' => 'enable-beta-toggle',
					'value' => '1',
				] ),
				[
					'label' => new OOUI\LabelWidget( [
						'label' => new OOUI\HtmlSnippet(
							Html::openElement( 'div', [] ) .
							Html::element( 'strong', [],
								$this->msg( 'mobile-frontend-settings-beta' )->parse() ) .
							Html::element( 'div', [ 'class' => 'option-description' ],
								$this->msg( 'mobile-frontend-opt-in-explain' )->parse()
							) .
							Html::closeElement( 'div' )
						)
					] ),
					'align' => 'left',
				]
			);

.oo-ui-fieldLayout.oo-ui-labelElement.oo-ui-fieldLayout-align-left > .oo-ui-fieldLayout-body > .oo-ui-fieldLayout-field, .oo-ui-fieldLayout.oo-ui-labelElement.oo-ui-fieldLayout-align-right > .oo-ui-fieldLayout-body > .oo-ui-fieldLayout-field {
    width: 60%;
}
.oo-ui-fieldLayout.oo-ui-labelElement.oo-ui-fieldLayout-align-left > .oo-ui-fieldLayout-body > .oo-ui-fieldLayout-header, .oo-ui-fieldLayout.oo-ui-labelElement.oo-ui-fieldLayout-align-right > .oo-ui-fieldLayout-body > .oo-ui-fieldLayout-header {
    width: 35%;
    margin-right: 5%;
}