To be able to change your ubuntu theme, you’ll need to isntall Tweaks.

In Debian and Ubuntu(18.04 or newer), install Tweaks by using the following command:

sudo apt-get install gnome-tweaks

For older Debian and Ubuntu versions, install GNOME Tweaks Tool using the command below:

sudo apt-get install gnome-tweak-tool

To be able to change the GNOME Shell theme, you’ll need the User Theme GNOME Shell extension.

sudo apt-get install gnome-shell-extension

You can download a theme from here https://www.gnome-look.org/p/1252100/ and then unzip the file and place it in your themes directory ~/.themes/ or /usr/share/themes, then use tweak select the theme you want to use.

communitheme is the theme that I like more, you can install it use command sudo snap install communitheme

it’s github is https://github.com/ubuntu/yaru