Hi there, the following will center the widget titles and allow you to change the color. I used a red, just for demonstration purposes. Go to Appearance > Customize > CSS, delete all the informational text in that window, and paste in the following custom CSS.
On the header, what you would do is to upload the image to your media library, get the URL of that image and replace URL_OF_IMAGE between the quote marks with the URL of your new image in the CSS below. The existing header image is 180px in height, and the following would allow you to increase that to 250px. If you want it taller than that, we will have to make some adjustments.
background: url("URL_OF_IMAGE") no-repeat scroll center top rgba(0, 0, 0, 0);