As you sent me some code, i thought you can write code. Sorry for that.
Theme can override this plugin css so it may also override Appearance settings.

You can use following code

.fep-button{
    background-color: #ffffff !important;
    color: #5033679c !important;
}
.fep-button:hover,.fep-button-active{
    background-color: #5033679c !important;
    color: #ffffff !important;
}

Then clear your cache (browser and website) and test.
Please let me know.