Don't know what exactly PostCSS is but with a JavaScript addon it would probably be wiser to inject the custom css directly into the shadow dom instance if possible and avoiding such hacks.
Also, by the way, when JavaScript addons get involved: userChrome.css is applied quite unfortunate in the css cascade. It gets low priority that is why they are usually full of !important rules. With JavaScript it is possible to add custom css instead as so called author stylesheet which makes it easier to override default styles. (never tried it myself)
Also, by the way, when JavaScript addons get involved: userChrome.css is applied quite unfortunate in the css cascade. It gets low priority that is why they are usually full of !important rules. With JavaScript it is possible to add custom css instead as so called author stylesheet which makes it easier to override default styles. (never tried it myself)
https://old.reddit.com/r/FirefoxCSS/comments/msoqte/how_can_...