There is a dark theme for plasma, but is there any application that can invert the colour scheme completely. i.e. make whites black and vice versa, etc?

Yes. There's an Invert effect you can enable and fiddle with in System Settings -> Desktop Behavior -> Desktop Effects -> Invert

Looks like it only does direct RGB inversion, which means the colors will be inverted (e.g. red <-> teal, yellow <-> purple, etc.) What'd be nice is a value inversion which preserves the hues of the colors and just makes light colors dark and dark colors light.