Hi callum-ideamapper
I'm sorry for this lateness. Basically, T4 Builder content will be rendered inside the body tag of a page so when you add the custom css for body tag or html tag (the tags outside of the body) inside the builder, it will not have affect. If you add custom css for the div inside the body like you mentioned:
#iuu7j {
background-color:#ff0000;
}
it will work properly.
Please try to add the custom css for body, html in custom css file of Joomla template you're using.
Regards