Η team of Ubuntu has added a new theme to Ubuntu 20.04 LTS. The 19th week from development its almost over. Before the other upgrades, the new Ubuntu comes with new ones issues. In the photo below you can see that there is now a third choice in the themes. The new theme combines a bright background with dark windows.
If you are new to this operating system , you should know that you need to install gnome-tweak-tool first, because the system itself doesn't gives the ability to change it theme.
You can install gnome-tweks by typing the following commands into a terminal:
sudo apt update
sudo apt install gnome-tweak-tool