Error in control icon background.

Hello, I am in the process of installing and using the Guestbook plugin. So far, I really like its possibilities for my intended use. I am using a theme called Press Book Masonry Dark” which is based on a predominantly black (#000000) background on all pages and posts. When the “write a new entry” button is clicked and the entry form loads, part of the text field where the “content” is typed is obstructed with a black shape. This black shape covers icons that indicate “bold”, “Italic” “bulleted list” “link” designations, etc. I am NOT a code writer so I simply do not know how to correct this which I assume is a fix to the CSS. The rest of that top bar has a #E7EBEE background which is an appropriate color for the black type or font. Please advise as to how I can fix this. Thanks in advance for your assistance.

The page I need help with: [log in to see the link]