If you've selected the Subtitle Theme in the Player Builder, but when previewing the video, the CC (Closed Caption) toggle cannot be enabled you'll need to also select the Subtitle module as a Player Add-on.
This is done by clicking the "More Add-ons" button from the player page in the Player Builder. Then select "Subtitles". The subtitles theme and subtitles modules/add-on must be used together for subtitles function properly.