Custom CSS

I want to add a custom CSS to my mashup. I tried to follow this document:

https://support.ptc.com/help/thingworx_hc/thingworx_8_hc/index.html#page/ThingWorx/Help/Mashup_Builder/Mashups/CustomCSSTutorial.html

But this does not work because I cannot see a "CustomCSS" tab. I want to write a small css code just to rotate the mashup 90 deg. Code should be =  transform: {rotate(90deg);} but I donot know where to write it in the composer.

Thank you