I am trying to modify the CSS of the website http://www.baomoi.com for my grandmother. I\'d like to modify the CSS for easier readability for her and to make it more minimal
the GreaseMonkey plugin for Firefox (or TamperMonkey plugin for Chrome) may be ideal for what you're trying to accomplish you can specify a specific site and which script should run on that site.
There are several community scripts available on Userscripts.org: Power-ups for your browser or like you've done create your own.
EDIT:
use csspivot.com (edit: dead url)
you can rewrite a site's CSS and re-access it with a URL that's saved on csspivot.