some css for styling arrows in structure panel

style.css

/* to make them more visible (default was 0.4) */
#bricks-structure .structure-item svg {
  opacity: 1;
}

/* to change the color of the arrows */
#bricks-structure .toggle svg {
 color: red;
}

/* to change the color on hover */
#bricks-structure .toggle svg:hover {
 color: blue;
}

/* if you want to change for arrows when inside components only */
#bricks-structure.component .structure-item svg {
  opacity: 1;
  color: green;
}