I dont think this is possible. You can only change the background color of the page and form as well as the layout sizes, but not the theme itself.

Hi,

In addition to what Prineel has mentioned If you would like the capability to be added in the future you may log a feature request at https://nintex.uservoice.com/

Everyone that works in your office can vote on the idea to get more attention from the developers 

The link provided is a forum wherein you can log a new feature request or vote for other user's request so that they can be considered for future updates.

Cheers,

Kate

Hi both and thanks, yes I will open a request for future improvements. The only idea that I have now is to create 2 themes (one dark and one light) than create 2 forms calling the same views but the 2 forms will have a different theme, so, it's a work around but it's working! Both forms will have a button on the top of the page with the switch between "themes".

Thanks for the help anyway

With a bit of tweaking one can set the theme in a parameter. So what you can do is have the dropdown list redirect to the same page but pass the system name of the theme. The parameter name is _theme, i.e. [url]/USTS.CM.Landing?_theme=lithium. 

Disclaimer, I haven't tested this with custom themes as I do not have any in my environment, but it works for out of the box themes.