Ascent theme – Issues changing floating header color

I apologize if this has been answered in the past, but this is my first go with a WP site and I have been searching for an answer to this question with no avail so far. I am also very new to PHP and have limited CSS experience.

I recently created a new website, https://www.phoenixdownradio.com and am using the Ascent theme. I have had some luck making changes to many portions of the theme to make it fit the needs of the site, but I cannot get the floating header background to change from white. Any assistance would be greatly appreciated.