Navigation Bar, Sidebar, display thumbnails

After installing the Highend theme I noticed that I cannot change the border color to the navigation bar, and the width of my sidebar is smaller than the demo. (With the border color, I change this under Appearance > customize, but no matter what color I choose it stays white.) How do I fix these? Also, I would like to have my posts page display a thumbnail of a picture, and not a cropped full size version of an image. How can I change this? Thank you!