Hi there!

Thanks for your query.

Custom CSS needed. You can try inspect and find the code snippets:

https://www.joomshaper.com/blog/which-line-of-css-i-have-to-edit-code-inspector-in-action

Here is a sample custom code you can try in your custom CSS to change the background color:

.top-search-input-wrap {
    background-color: red;
}

The following screenshot may help too:

Best Regards