Creating a themed UI - Any tips, pointers, ideas?

Hi. I want to create a themed UI, so change the look of widgets depending on the setting etc, but not having to duplicate widgets. The game will have multiple windows on screen a lot of the time, and so I was thinking of different modes, a night mode, a day mode, combat mode etc. But all I really want to do is change the images used for buttons, backgrounds, borders and so on. Picture attached for the extra 1000 words.

What would be a good way to go about this?