Just ... Scroll ... Down ... Click where it says "read more" or "show more replies"
You're human; THE most adaptable creature known. Adapt!
I'm not saying that twitter UX is perfect, or even good. I AM saying that it is usable.
Once you scroll up, it sort of makes sense -- each tweet with a line connecting user icons but then suddenly the expanded tweet thread has the main tweet in a larger font, then the "retweet/like" controls below it, THEN another line of smaller-font tweets that comprise the thread. Then you get some limited number and have to click "more" for more.
The monochrome of it all reminds me of when GMail got rid of the very helpful colors-for-threads and went to grey on grey on grey.
It's not visually apparent at all.
I'm saying it's usable.
I'm saying that complaining about it makes people look like they think they're royalty who need everything just so or their whole day is ruined... And now they can't tell the butler to take Poopsie for a walk because they're so shaken by the experience.