how to set a editable qwebview font size?

i load a local html file use qwebview .
i want to edit the html content .so i set qwebpage setContentEditable(true).
but i found i can't set the input text font size .
the code is:
QWebView webViewWidget ;

int fontSize = 20 ;

QFont ft = webViewWidget ->font();

ft.setPointSize(fontSize);
webViewWidget ->setFont(ft);

webViewWidget ->fsettings()->setFontSize(QWebSettings::DefaultFontSize,fontSiz e);
but the text i input in webview widget is also the same size .
can someone tell me how to solve this problem