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ServerName theme2.example.com
AliasMatch ^/ui/theme/(.*)$ /var/www/html/ui/theme2/$1
AliasMatch ^/ui/config/client-config.json /var/www/html/ui/config/theme2.json

Abiquo.properties configuration

You only need to ensure that abiquo.server.api.location property points to the common API URL:

# grep api.location /opt/abiquo/config/abiquo.properties
abiquo.server.api.location = https://api.example.com/api

grep api.location /opt/abiquo/config/abiquo.properties
abiquo.server.api.location = https://themeapi.bcn.abiquo.com/api

# tree /etc/httpd/conf.d/
/etc/httpd/conf.d/
??? api.conf
??? ssl.conf
??? theme1.conf
??? theme2.conf

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