That’s made with an image, so the only way to change its color is by editing the image itself. I made one with a white background here. You can right click it and save it to your computer:
After you’ve done that, go to the Media section in your WordPress control panel and select Add New. Upload the image and click the edit link once the upload is finished.
Look on the right hand side of the edit screen. You’ll see a box labeled File URL. Copy that URL from the box.
Next go to the Styling tab in the Theme Options page. Scroll down to the Custom CSS box and paste this:
.header-sidebar .widget_twitter,
.header-sidebar .latest-tweets {
background: url(https://yoursite.com/image.png) no-repeat left top;
}
Then replace the http://yoursite.com/image.png with the URL you copied earlier. Update your settings and that should change out the silver background for a white one.
If you have any trouble, let me know,
Bill