You should start tinkering with embedded webservers then (civetweb, etc)! Even embedded devs should learn web tech because often browsers are the easiest way to interface with router/modem settings, IoT settings, etc.

In this case "##" is uBlock filter syntax. Basically "news.ycombinator.com##html:style(filter:invert(100%) hue-rotate(180deg))" means "if the host matches news.ycombinator.com inject the following CSS"

Here's the docs if you want to rtfm: https://github.com/gorhill/uBlock/wiki/Dashboard:-My-filters

In this case I believe we are dealing with "cosmetic filters": https://github.com/gorhill/uBlock/wiki/Procedural-cosmetic-f...