Are you still running Raspbian Jessie? That's not the current default window border, gtk theme, or menu appearance.
Upgrading to Raspbian Buster will solve your problem.
BTW this is what Raspbian Buster looks like:

- pi3-10.jpg (57.98 KiB) Viewed 255 times
Want to make your own Raspbian image?
- Hard way: Mess with confusing terminal commands.
- Easy way: Let Pi-Power-Tools do it all for you!
Boot your image in a
VM,
Flash from the Internet at top speed,
Mount (to drop files in),
Edit partitions,
Shrink/Expand,
Repair, and more. Enjoy!
https://github.com/Botspot/Pi-Power-Tools