indent.admin.css

.ckeditor5-toolbar-button-indent {
  background-image: url(../icons/indent.svg);
}

.ckeditor5-toolbar-button-outdent {
  background-image: url(../icons/outdent.svg);
}

File

core/modules/ckeditor5/css/indent.admin.css

View source 
  1. .ckeditor5-toolbar-button-indent {

  2.   background-image: url(../icons/indent.svg);

  3. }

  4. 

  5. .ckeditor5-toolbar-button-outdent {

  6.   background-image: url(../icons/outdent.svg);

  7. }