Glad to hear that you like the add-on.

Unfortunately, there are some websites (Twitch is a good example) that have taken scrollbars a step further. Instead of using an actual scrollbar, these websites have designed custom components that look and behave similar to scrollbars, but they are fully custom. Since these are not actually considered "scrollbars" by the browser, it's not currently possible for me to make any modifications to these scrollbars.

As for the transparency issue, some websites have not set the background of the page (meaning it stays white) and instead have a large panel that they have set to be the width of the screen. The width of the screen does not include the scrollbar, so the white background hiding below shows through. Much like the issue above, there's no way for me to fix these either.