Are you talking about what I have highlighted red?
View attachment 74062
EDIT:
If so, try (in your EXTRA.css)
Code:
#redactor_media_link {
background-color:blue;
color:white;
}
where the background color is what you want. You may need to use an !important declaration after the color.
Default mine looks like this
View attachment 74063
Now it looks like this
View attachment 74064