The Thrive theme features decorative graphics that change colour based on the colour scheme that is in use. They look like this:
These graphics are displayed by default, but you can remove them if you wish.
To remove the graphics, open up the Colours and Design Featured tab in the customiser.
Scroll to the bottom of this tab and here you will see the Graphics section and the option to show the graphics. To hide the graphics, simply untick the box next to this option.