I have several blog posts about themes and theme color.
Smart theme has a combination of a background color + a JPEG image (find that weird, but I didn't create that theme).
Go into the Master slide panel, to the main master slide and look at the Properties panel:

Depending on the size of the project, the JPEG is smaller than the slide.
The Color you see (red arrow) is color 6 of the Theme colors palette.

You could edit the theme colors palette.
As for the JPEG, you can edit it with the pencil icon (see first screenshot), maybe just increasing the Brightness?
If the editing is not sufficient, you can find that JPEG in the Library and edit it with any graphical application. I really wonder why they don't use a shape, but ....