cascade VS elementaryos-firefox-theme

You need to learn Objective-C and Java basics if you want to create real things 😎 Try to create some Native Components and try to use Native Libraries to make your app looks perfect 👍

Instead, take a look at this https://github.com/saket/cascade

for the theming in Firefox i currently use https://github.com/Zonnev/elementaryos-firefox-theme/tree/elementaryos-firefox-theme but I sometimes switch back to standard look ...in combination with tweaks it adapts the dark mode (the ubuntu repo firefox installed with apt)

You can try this Firefox theme for a better integration :)

I installed this theme: https://github.com/Zonnev/elementaryos-firefox-theme/tree/elementaryos-odin-firefox-theme

Yeah, this triggers me too. I remember having the same problem on KDE though, so it's most likely a Firefox thing. You can use this custom eOS theme for Firefox if it's bothering you enough GitHub - Zonnev/elementaryos-firefox-theme: This theme for Firefox supports all the window buttons layouts from Tweaks and it blends into the elementary OS user interface.

As in the title, I have weird black boxes behind the modals (e.g. bookmarks on the screenshot above) on Firefox. Initially I thought it was related to custom theme but after removing it and reverting toolkit.legacyUserProfileCustomizations.stylesheets, problem still exists. Any idea how to fix it?

By default no but thanks to the zonnev we have this: https://github.com/Zonnev/elementaryos-firefox-theme

If you are running the early builds of eOS 6 Odin you can install Firefox in there and add my theme for Odin. It's not recommended to install the Odin theme in eOS Hera since it won't look good because of double window borders and larger rounded window corners. For some reason in eOS 6 Odin, Firefox doesn't get window borders, so I added them manually with the style sheet.

This is a tweak of the original theme by Zonnev. I'm still working on some details, then I'll put the userChrome on Github as well.