Fullscreen Slider Button

it is the place where you have entered the text for that button.:
Open the slide itself – a new popup then will show the options for that slide. Click on the advanced tab. There should be a link settings – on the right side of the label there is the color of that button. Sadly there is no custom color with color picker option – but a lot of alternatives to your given choice.
If you have set the on styling tab to show “Use first slides caption as permanent caption” then you had to open the first slide.
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If that’s not enough, you would have to set the background color via CSS.
better way is to do it via a custom class on your slider – but try:

#top.page-id-1104 .avia-slideshow .avia-slideshow-button {
  background-color: #026538;
}
#top.page-id-1104 .avia-slideshow .avia-slideshow-button:hover {
  background-color: #719430;
}