Site behind a reverse proxy - subfolder
You just need to reverse proxy all the asset directories like here (this proxies site preosasde via real site proreos):

ServerName preosasde

ProxyPass /themes/ http://proreos/themes/
ProxyPassReverse /themes/ http://proreos/themes/

ProxyPass /core/ http://proreos/core/
ProxyPassReverse /core/ http://proreos/core/

ProxyPass /vendor/ http://proreos/vendor/
ProxyPassReverse /vendor/ http://proreos/vendor/

ProxyPass /sites/ http://proreos/sites/
ProxyPassReverse /sites/ http://proreos/sites/

ProxyPass / http://proreos/asde/
ProxyPassReverse / http://proreos/asde/


Take care about the order of that entries and be aware that you will loose your users credentials (cause of the different domain).