Need help with Charles Proxy (Stupid qn)

Hi, is there any way to modify things in that are in XML
I have this: [Only registered and activated users can see links. Click Here To Register...]

and I'd like to change the color, but when I click on Edit appears a different thing, just the Host, User-Agent, Referer, Cookie...

I just wanna edit the XML, is it possible?

PS: First time using Charles Proxy

Should be possible, but I see hexvalues there. Try Wireshark if Charles isn't powerful enough. :)