Update the template styling for public pages

[christianlupus]

There has been an announce on https://help.nextcloud.com/t/guest-public-page-changes-in-master-from-nc-25-on/142838:

Hello app dev,

For [NC] 25, we have significantly changed the style of the guest/login pages (see Redesign guest pages for better accessibility by CarlSchwan · Pull Request #33265 · nextcloud/server · GitHub 2). If your app is providing some public pages, please make sure to adapt it to the new style.

In practice, it only requires, wrapping your existing UI with the following code:

<div class="guest-box">
     // existing code
</div>

We should fix your styles in the guest app accordingly.