> I’m an embedded dev, so websites and web tech are a black box to me

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...

TIL that ublock lets you inject CSS. I've been using tampermonkey for that (most recently to fix the mspace tags on ncatlab.org).

> means "if the host matches news.ycombinator.com inject the following CSS"

Well I wouldn't word it like that.

    youtube.com##.ytp-cards-teaser

Hides elements with the selector ".ytp-cards-teaser", so ## doesn't represent "inject the followong CSS".

This is a helpful link: https://github.com/gorhill/uBlock/wiki/Introduction-to-basic...

## represents a cosmetic filter. Though I can't seem to figure out what #$# is still but I know that's also possible to use