How much is customizable in the gantt?


I know there seems to be a few themes supplied for the gantt, can new ones be added easily, all done through css?


yes, all skins can be amended via css. And you can create a custom one on base of any existing.
In addition to css, some values should be also specified in JS component configuration, as they are used for rendering the component. Such values also can be changed.
For example:

gantt.config.grid_width = 200; gantt.config.grid_width.row_height = 27;