I had a similar issue on my fresh installation, not in ACP but in the frontend for me. I created a ticket but they told me that it might be related to CloudFlare configuration.
I checked the source code and found that the CSS variables were gone and therefore all variables were undefined. I don't know if this is a bug in 2.3 or related to CloudFlare configuration.
I reinstalled XenForo and it works as expected now.