The CSS code looks good to me!
I have only two small remarks:
- When you add the declaration ‘background-color: #000000’ to the class ‘button’ all buttons on the website get a black background. If you only want to style the button in the cookies pop-up, use the element selector of the developer tools (F12) and try to find out which class is attached to the cookies pop-up button. Add the background color to that class.
- If the added CSS code doesn’t have any effect, try adding !important to the declaration.