Hi Mark,
The custom interface option is for Flash only i'm sure, the one you want to use is the Stylesheet one... with this you can override aspects of the HTML5 layout.
For the Bootstrap layout your optional properties have two options that will be appropriate, the Styles one that allows you to insert inline style and the Stylesheet one that allows you to attach a ready mae stylesheet. This can override or augment the styles that are there already, adding in the features that you require, although upon testing this morning I can't seem to be able to get them to work... will email the others to see what is going on...
Regards,
John