Manipulating the CSS of the COLVIS button?

SKC

I'm currently using the JQuery Datatable for my .Net MVC Web-app.
I've added the default colvis button in the UI.

Here's the UI when the button is clicked (have attached the image)

In the above image I have deselected the first 3 columns from being displayed in table. But it's a bit difficult to differentiate between the selected and non-selected.

I have 2 questions :
1.Can we manipulate the CSS for these. Like changing the Graadient or differentiating them by color (light colour for deselected one's and dark for selected ones? How can we do that,if it's possible?
2.Can we add scrollbar to this list? As we need to scroll the entire page to view the complete list?

I was able to manipulate the CSS of colvis button by using the following code :

.dataTables_wrapper .dt-buttons { //css code here}

But I'm unable to reach to the solution to manipulate the coloumn name buttons when the 'colvis' button is clicked.

I know that colvis is now Legacy but I need know this.
Thanks in advance