Hugo - Add custom CSS file

It is not a good idea to modify Hugo theme's folder if you want future support and upgrade. Instead, add your own css file in params.toml. For example, I created a new folder called assets/css/ from the root level and add custom.css file to it. Then, I added the following line in params.toml file

...
custom_css = ["css/custom.css", "scss/custom.scss", ...]
...