search api not properly themed

Problem/Motivation

Using the search api module, the theming make it almost impossible to add nested fields for indexing

Steps to reproduce

-define a server and an index , using the search api module
-add fields for indexing
-the major problem is when wanting to index nested fields ( from nested paragraphs in my case ), it is then required to click the + bottom many times and the design makes it almost impossible to select the proper fields.

/admin/config/search/search-api/index/my_index/fields