Hello 
Here is an example for how to change the color of those buttons.
.banner-box {
.button-container {
.btn {
.d-icon,
.d-button-label {
color: var(--tertiary); // change this to the desired color
}
// hover and focus color
.discourse-no-touch & {
&:hover,
&:focus {
.d-icon,
.d-button-label {
color: var(--tertiary-hover); // change this to the desired color
}
}
}
}
}
}