Fresh install of Beta 19 - Forum CSS works now
 

Our environment: Windows 8 with IIS 8 and Web Matrix 4.0 with fresh install of vb5_connect

Initially we installed PHP 5.4.0 using Web Matrix and our forums page would show only text and no styles.
We fixed it by installing PHP 5.3.19 using Web Matrix 4.0 and removing PHP 5.4.0.
Hope this helps.