In case anyone finds this in the future, this behavior is actually natively supported with the cellStyle option set to flat. From the documentation:
flat - No styling applied. Data is shown in default MM styling
The default for this option is mimic which copies exactly what you have in your existing sheet. If you didn’t want to use flat, you could still use mimic but change your sheet to match the MM (background color black, text color #fff, #666, or #999).
If use the custom css be aware that it will have undefined behavior if you try to change other options in the future.
Thanks for using the module!