As well as built in themes it comes with extensive documentation on the CSS classes as well as overrideable SASS variables for all themes to make customising as easy as possible, more information can be found in the styling documentation:
http://tabulator.info/docs/4.1/style
It also comes with a number of themes to help match other UI frameworks. See the themeing examples here: http://tabulator.info/examples/4.1#theming
As well as built in themes it comes with extensive documentation on the CSS classes as well as overrideable SASS variables for all themes to make customising as easy as possible, more information can be found in the styling documentation: http://tabulator.info/docs/4.1/style