basic.admin.css

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

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

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

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

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

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

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

File

core/modules/ckeditor5/css/basic.admin.css

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

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

  3. }

  4. 

  5. .ckeditor5-toolbar-button-italic {

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

  7. }

  8. 

  9. .ckeditor5-toolbar-button-underline {

  10.   background-image: url(../icons/underline.svg);

  11. }

  12. 

  13. .ckeditor5-toolbar-button-code {

  14.   background-image: url(../icons/code.svg);

  15. }

  16. 

  17. .ckeditor5-toolbar-button-strikethrough {

  18.   background-image: url(../icons/strikethrough.svg);

  19. }

  20. 

  21. .ckeditor5-toolbar-button-subscript {

  22.   background-image: url(../icons/subscript.svg);

  23. }

  24. 

  25. .ckeditor5-toolbar-button-superscript {

  26.   background-image: url(../icons/superscript.svg);

  27. }