You can use then view helpers of this extension to effectively add a contrast toogle to your page.
Add the namespace like {namespace etc=ElementareTeilchen\Contrast\ViewHelpers}
Place this i.e. in the body tag or any wrapper html element
<body class="yourClass {etc:contrast.toggleCssClass(className:'t-contrast')}">
Add you contrast link like this:
<etc:contrast.toggleLink class="contrast__link link-list__item__link" addQueryString="false">
<f:translate key="page.contrastView" extensionName="contrast" />
</etc:contrast.toggleLink>