To modify the CSS properties of HTML elements, we need to get their selectors, which could be tags or identifiers like classes or IDs, so we can actually target them in CSS. The most common and arguably correct way is to use classes (unless there is no class because, in our surveys, we cannot modify HTML).
The easiest way is to go into the Test tab of a Survey -> Right click on anything you want to modify -> Inspect
Classes used on the button element we selected are 'btn' and 'btn-success'. While both would do the job, it's better to use the more specific one, in this case 'btn-success', if you want to modify only one specific element. The more general class, like 'btn', could be used in all buttons and could be used to modify the CSS properties of all the buttons at the same time, if all the buttons shared the same CSS property.
Now that we know the element's selector, we can go back to Design -> CSS and modify some styles of the button. I decided to give it a black color. There are many ways to do it, including typing out 'black', but I decided to go with the Hexadecimal color code representation, #000.
And here is our result:
CSS can be a very powerful tool in the right hands. However, keep in mind that surveys are not primarily designed to support heavy CSS and JS applications.
Custom code is out of scope for support to assist with or debug.