Tidy up the messy CSS

[koheiw]

The Plato theme is good, but its CSS is messy in terms of structure and format. There are too many overwriting, conflicting and unused instructions that make maintenance of the website difficult. We need to commission this task to someone who regularly do CSS coding.

https://github.com/quanteda/quanteda.org/blob/master/themes/quanteda/static/css/overwrite.css
https://github.com/quanteda/quanteda.org/blob/master/themes/quanteda/static/css/style.css