We cannot customize the appearance of the 608 Caption track. It will always show up with default settings due to the limitations of the export. The appearance of the captions on the 608 track will always be dictated by the player.

The 708 Caption track offers more options since it was specifically invented to allow you to customize the look of your captions.  Customization options are still a bit limited (you can’t be super specific with exactly how transparent you want the boxing to be, for example), but they are there. The big caveat is that we can’t guarantee how well the customized track will work in specific workflows/players. The client will have to do some testing.

Additionally, it’s important to note that the customization would have to be done on the client’s end. If we’re providing an SCC, that SCC will be exported with our defaults. Formatting would have to be applied upon export of the final embedded video. In MacCaption, for example, the client could go to Captions > Encode Preferences and find a menu with the options, select what they wanted, then export the newly styled captions. 

An MCC is designed for even more customization of the 708 track and will work in Premiere, so the client may want to try that if they’re handling the embedding and customization themselves.