I will say following Yihui's suggestion:
R's default theme of the HTML help pages is too plain for me to read, but we can easily modify the theme, which is essentially a CSS file. You can find the file under:
file.path(R.home('doc'), 'html', 'R.css')
Simply replace this file with my version of [R.css](https://github.com/yihui/configuration/raw/master/R.css) (1K) which looks like:
Of course you can design your own `R.css` if you know CSS.