Can you guess which DE's theme does my windows use? And how to recover my Ubuntu to fully use the GNOME theme?

Installing (and uninstalling) alternative desktops will often mess up the default environment. There are a couple of options that may restore the default Gnome desktop but, be warned, this will restore stock settings. Any customisations that you have made will be wiped clear.

Option 1: Reset Gnome

The simplest is to reset the Gnome Desktop settings:

0. Open Terminal (if it's not already open)
1. Reset Gnome:

   dconf reset -f /org/gnome/
   

   ^{Note: Do not do this via SSH. You must be on the machine itself.}

Option 2: Reinstall the Desktop Environment

Sometimes a reinstallation of ubuntu-desktop is necessary:

0. Open Terminal (if it's not already open)
1. Reinstall the Desktop Environment:

   sudo apt install --reinstall ubuntu-desktop
   

One of these two ought to get you back to a default Ubuntu Gnome desktop.