UI Builder. Styling underlying text input component (#unity-text-input)

I want to style inner component to make my input look better (match design) without background and with bottom border. Is there a way to style that guy?

Currently I cannot change any styles of that node in ui builder, because each entry is disabled. I cannot also add custom css classes in it. What do you advice?

 

Yep, you can use USS selectors to change the styles of child elements. See this similar thread:
https://forum.unity.com/threads/uielements-text-field-changing-child-elements.789656/#post-5260388