在webView的delegate回调方法    - (void)webViewDidFinishLoad:(UIWebView *)webView;中写上一下语句即可

//字体大小

[webView stringByEvaluatingJavaScriptFromString:@"document.getElementsByTagName('body')[0].style.webkitTextSizeAdjust= '330%'"];

//字体颜色

[webView stringByEvaluatingJavaScriptFromString:@"document.getElementsByTagName('body')[0].style.webkitTextFillColor= 'gray'"];

//页面背景色

[webView stringByEvaluatingJavaScriptFromString:@"document.getElementsByTagName('body')[0].style.background='#2E2E2E'"];

或者：

UIFont * fontSize = [UIFont systemFontOfSize:16]; 

UIColor *fontColor = [UIColorFromRGB(0x666666)];

NSString *jsString = [[NSString alloc] initWithFormat:@"document.body.style.fontSize=%f;document.body.style.color=%@",fontSize,fontColor];

[webView stringByEvaluatingJavaScriptFromString:jsString];