In the Designer there is no way to customize the color of the title bar. You do have the option of using the theme files to set the color of the title bar, as well as any other style options you prefer.
.ia_popup_header {
background-color: #00FF00;
}
I recommend being on 8.0.13 or newer to set this up. Yo should also read the following threads/posts as they provide a walkthrough on what you need to do: