Previously adding custom css files would require the end user to copy
the header or footer partial into their overrides. They would then
need to maintain that partial into the future.
This config allows a user to provide a list of custom css files to load
into the theme, meaning they do not need to clone the partial.
```
[params]
custom_css = ["css/foo.css", "css/bar.css"]
```
Inspiration taken from https://discourse.gohugo.io/t/how-to-override-css-classes-with-hugo/3033/4