CB Activity does not look dark like that by default. Especially if using protostar template. Are you using a custom CB template? Switch to default CB template, default CB Activity template, and default Joomla protostar template. You have a CSS conflict cutting off the dropdown. If you generated a custom CB template with CB Template Changer be sure to re-generate your template after every CB Template Changer update as we do update the SCSS files responsible for generating the CSS with new releases.
If issue persists after doing the above please PM backend super administrator login credentials and will take a look.