Linux Themes
Here is a simple sharing about how to modify the themes in Ubuntu.
Preparation
Before changing, the first is to install 'gnome-tweaks' & 'gnome-tweak-tool'. You can use 'apt/apt-get' to install.
Download and Set
Which theme to choose is your freedom. You can choose from here:
https://itsfoss.com/best-gtk-themes/
and put themes in '.icons', '.themes'.
Finally, choose themes in 'tweaks'.
Linux Themes
http://blog.chivier.site/2020-09-18/6ae59753b715/