Hello, I wonder if anyone knows and is so kind to let me know how to replace the link "Mark site read" at the top right corner of the screen with a custom stream. (However, I'd be happy to keep it where it is at the bottom of the page)

Anyone... please?

You'll have to edit the breadcrumb template and replace this 

<a data-action="markSiteRead" class='ipsType_light' data-controller="core.front.core.markRead" href='{url="app=core&module=system&controller=markread" seoTemplate="mark_site_as_read" csrf="true"}' title='{lang="mark_site_read"}' data-ipsTooltip><i class='fa fa-check'></i> {lang="mark_site_read_button"}</a>

with your new link

Thanks Daniel, yes, that's as far as I got, but I assume there's more to change than the actual link to the stream. Anyway, I can take it from there and try the rest by trial and error. Cheers.