When your mouse hovers over two themes at the same time and exits the modal, the off-hover event only fires for one, leaving the second theme hovered over selected and in preview mode.
Two options:
make the selectable area smaller so the mouse hover event doesn't overlap.
Edit the event or setup a new one to fire and clear the preview stylesheet once the mouse is outside the theme modal.
When your mouse hovers over two themes at the same time and exits the modal, the off-hover event only fires for one, leaving the second theme hovered over selected and in preview mode.
Two options: