Testimonial post uses post format „Quote“ and it is styled differently. For „quotes“ posts it is possible to change colors from inside WordPress customizer. Color change will than effect all the „quote“ posts and also other text and backgrounds with the same color as testimonial box…
Anariel Design – About Quote Format Post, 2013
Post Format: Standard Sticky
This theme uses WordPress feature called „Post Formats“. Using „Post Formats“ posts can be styled differently and independently of each other. This post, for example, is a „standard post“ format that is marked as „sticky post“.
„Sticky“ post always comes as a first post and it is usually used to highlight an important post.
More about „sticky“ post can be found here : codex.wordpress.org/sticky-posts.