See Following Working Example:
Suppose you have Following Iframe
Now you just need to add a script code which will add the css file in iframe.
What will do above code
Whenever iframe load in browser, "/css/myiframecss.css" file will will be added in the head of iframe.
You just need to add all the css inside "/css/myiframecss.css"
Note: Iframe and website must have same Domain, Port and protocol.