I'm just learning NTWS 5 myself, but I downloaded a custom theme, and extracted the scripts to the web\scripts folder, and the css to the styles folder. Then I renamed the original jquery-ui-1.8.custom.css to jquery-ui-1.8.custom.cssBAK, then renamed the new styles jquery-ui-1.8.2.custom.css to just jquery-ui-1.8.custom.css, and it seems to work fine.