I just upgraded to 5.8.0 and quickly noticed that my contribution pages didn't look the same. Using an html inspector the page is using just css from the joomla template and not civicrm.css. Looking at the source on the contributions page I see:
link href="/components/com_civicrm/civicrm/css/civicrm.css?r=LSlvr" rel="stylesheet" type="text/css"
However that is NOT where the civicrm.css file is located at. Its at administrator/components/com_civicrm/civicrm/css.
I "fixed" it by creating a symbolic link in webroot/components/com_civicrm to the directory in the administrator directory. (this is not the right way to fix it, but it works now!)
Is this a known problem? Is it something with my resource urls or directory settings? I've been playing with them without success.