Dont think there is way to do that, also are you sure you want to do it? It breaks UI Google/Apple’s UI patterns which might be used to specific look of back buttons.
Ok thanks for your reply. Yes I wanted to place an “x” button on top right corner.
Some people might want to design the browser window similar to the rest of their app rather than native style. Not only talking about the back button, but also the slide in effect on iOS, font type etc.
So you might want to consider more customisation options for future releases.