here's a simple example for you.
it shows 2 different ways to control some element on a webview.
the example loads a simple webview from dirassets.
there is a sleep command for 10 seconds so you can settle down and look at the webview (i don't want you to miss the show
.)
the first example executes a javascript command to change the text color of an element directly. comment that one and uncomment the second one.
the second example executes a javascript function which does the same thing.
i believe either case could work for you.
if you leave both uncommented, you will only see the result of the second example.
note: there is a button on the webview. if you tap it, the color changes. understand that the webviewextras executejavascript command that runs a function (the second example) is clicking the button programatically, as if you had tapped the button.