Coursepress theme: Cannot change the link button color through color panel (Simplylearn)

We're making course-pages for customers, and want them to be able to customize all colors through the coursepress theme design panel. But I do not find the option of changing the blue button color in the image below.

Why is not this option in the design bar of the theme, and where do I change this? This color is vital for my customers to get the color scheme to work with their sites.