foundry/Data/modules/subfolder-indent/styles/style.css

/* the first number changes the width of the indentation 
var(--subfolder-indent, 12px) -> is a variable in the modules settings in Foundry VTT, default is 12px*/

/*The rgba changes the colour of the sub folder - note this will get overwritten if the Folder is edited in game */

.subdirectory 
{ 
  border-left: var(--subfolder-indent, 12px) solid rgba(255, 255, 255, 0.2);
}