The theme documentation provides a list of available visual names, cardNames and property names.
At this point some further explanation is needed for the hierarchy within the theme definition:
visualName corresponds to available PBI visuals like treeMap, card, columnChart,…
styleName (as of today I am not sure whereto this corresponds to PBI Desktop language.. :-)) maybe someone can further explain this to me
cardName corresponds to the formatting card/option within Power BI Desktop. Attention here: the name in the theme JSON file is defined different than the User Interface name + do not forget case-sensitivity! (i.e. general => General; categoryAxis => X-Axis, valueAxis => Y-Axis, ..). See the documentation for the rest of the mapping.
This is good news if it makes it easier for developers to write CVD-friendly reports.