File manager (File explorer) with expandable tree
Often the simple "Files" app is preinstalled in Ubuntu.
Check "Allow folders to be expanded"
How to change terminal theme
Unfortunately, obvious way does not work. In Ubuntu terminal, we click hamburger menu in the top right corner -> Preferences -> General -> Theme Variant -> and nothing changes...
We select "Dark", however the terminal is still light..
Instead, we need to go to Hamburger menu in the top right corner -> Preferences -> Profile name (in our case "unnamed") in the bottom left menu -> Colors -> Built-in schemes. Voila!