I have replicated the issue. The className is added to a cell but I cannot change any of its CSS properties (like a background color). I will investigate the case, report it and come back to you with details later today.
Firstly I used some hotColumn demo from our page and got the same result. But then I created an example from a scratch and was able to get the styling applied. Please check this demo https://codesandbox.io/s/declaring-column-settings-forked-kgl90 I’ve placed some comments in the CSS section.