if you go to visual elements list on the left of bubble editor there is one called button. by default it will change colour when you hover over it when you view the page in preview.
to change the colour that appears on hover, double click the element, go to Conditional and change it there.
If the element already has a style attributed on the first page after you double clicked on it, either click Removal Style or Edit Style (if you want these style settings to be used again elsewhere)