1: Its by default like this, you can not change it.
2: You can use this CSS in your custom.css file
.slide_thumb_wrap .flexslider .slides li:nth-child(1) .thumb-wrap .thumb-text {
background: rgb(14 93 140 / 80%) !important;
}
.slide_thumb_wrap .flexslider .slides li:nth-child(2) .thumb-wrap .thumb-text {
background: rgb(255 0 0 / 80%) !important;
}
.slide_thumb_wrap .flexslider .slides li:nth-child(3) .thumb-wrap .thumb-text {
background: rgb(14 255 0 / 80%) !important;
}
.slide_thumb_wrap .flexslider .slides li:nth-child(4) .thumb-wrap .thumb-text {
background: rgb(0 0 255 / 80%) !important;
}