Try this using appearance>customise>additional CSS.
It will produce white text and a blue button, or change to whatever colors you like. In the preview in WordPress only the color of the text changes but it’s fine when you view the actual website.
#cn-accept-cookie
{
color:white;
background-color: #31638c !important;
}
#cn-more-info
{
color:white;
background-color: #31638c !important;
}
