I posed this same question in Fabrikar's forums and this is what they said:
A Joomla template that supports the Joomla bootstrap framework (BS2 in Feb 2018)
The bootstrap template coming with Joomla is "Protostar". You can use Joomla's Beez3 template if you add com_fabrik to the "Components Requiring Bootstrap" in Beez3 "Advanced" tab.
If you want to use a BS3 or uikit template try https://github.com/Fabrik/Fabrik-Joomla-alt-layouts
See also http://fabrikar.com/forums/index.php?wiki/jlayouts/
If you have a non-bootstrap Joomla template you may try an extentions loading the bootstrap, e.g. http://digitaldisseny.com/en/joomla-extensions/twitter-bootstrap-plugin-joomla (not tested)
Does it make sense to you?