app/assets/stylesheets/application/components/filepicker.scss
button.fp__btn {
border-radius: 4px;
text-transform: uppercase;
background-color: $btn-success-bg;
border-color: $btn-success-border;
&:hover, &:focus, &:active {
background-color: darken($btn-success-bg, 5);
border-color: darken($btn-success-border, 5);
}
}