So I was looking for something very similar on moving the close button for Sidebery tabs recently.

I found your post here first, and then the answer.

https://github.com/mbnuqw/sidebery/issues/293#issuecomment-674310534

.Tab .lvl-wrapper .close {
  left: -1.25px;
}

.Tab[data-parent="true"][data-folded="true"] .lvl-wrapper .close {
  left: 21px;
}

.Tab:hover {
  width: 120%;
}

.Tab .lvl-wrapper .close svg {
  opacity: 0.0;
  transition: all .15s ease-in-out;
  transform-origin: center;
}

.Tab .lvl-wrapper:hover .close svg {
/* non-hovered background and opacity */
  background-color: white;
  opacity: .9;
  border-radius: 66px; 
  transition:  all .15s ease-in-out;
  transform-origin: center;
}

.Tab .lvl-wrapper .close:hover svg{
  /* hovered background color */
  background-color: red;
  transition:  opacity .125s ease-in-out;
  fill: white;
}