I tried too, in order to change the design or add CSS of InAppBrowser using insertCSS method, but there is no success. Don’t know why this is not working.
I have also asked InAppBrowser - possible to Customize? insertCss not working on the forum for this. But no reply.
But after that as per my requirement I able to only change the Navigation bar/ tool bar by tweaking into plugin file i.e. InAppBrowser.java.
Follow below link, check may this help you.