Hello Noemie!
I'm happy to help! I apologize for the disappointment. Once I understand the issues you're running into, I'll report them to my team for further investigating.
First, it sounds like you're using the Format Painter to change the style of quiz buttons. I've tested this using a Multiple Choice Graded Question. Here's what it looks like on my end: Link
Secondly, triggers are set to the Timeline if the Slide Advances setting is set to Automatically. If you want it set to when the user clicks, then select By User. Here's a screenshot of this setting.
If you see something different on your end, can you share a screen recording of the issue with us?