Hi! Yes, you can add custom-CSS to parts of your form if your form is embedded. Customizing the CSS is not possible if you use the direct link to a form.
Customizing CSS in Tripetto works a bit different than the global custom-CSS feature in WordPress. This article explains how to do this: https://tripetto.com/help/articles/how-to-use-custom-css-in-embedded-forms/
In your case you want to customize the Multiple choice question block. You can for example then do something like this: [data-block='tripetto-block-multiple-choice'] { button { background-color: red; } }
It depends how you embed your form (shortcode/Gutenberg editor/Elementor) where you can add that custom CSS. In each of the options it is a parameter/setting in the pertaining embed block.
At the moment it’s not planned to change this behavior of the unselected/selected state, but I will note your request for this on our backlog, so we can have a look if this is something we can improve in a future update.