The main repository has the source files for the core styling and the InvoicePlane base theme included. The minified CSS files will never be present in the repository as they are compiled from the source files. This is a best practice.
The downloadable zip from the repo (e.g. 1.5.9.zip) includes these compiled files as well as additional languages and so on.
If you want to change something in the core files or correct them, read further in the contribution guide. You need to have Node and NPM installed as well as Grunt CLI. Then you can compile the CSS files with grunt build.
Thank you @Kovah. Your response set me on my way to getting the solution.
For those that want easy instructions, on Ubuntu 18.04
Install Node and npm
The Ubuntu version lags behind the official LTS version and is on version 6.
You can install the current version using the nodesource ppa curl -sL https://deb.nodesource.com/setup_8.x | sudo -E bash - sudo apt-get install -y nodejs
Install grunt
(Note not grunt-cli) sudo npm install -g grunt
Run npm install and grunt dev
This is explained in the contributions file and link provided above by @Kovah.
The grunt dev runs as a service, so you will need a free terminal session, or run in a screen session, or run as a background task. npm install grunt dev