Use Bootstrap utility classes with slight SCSS modifications.

app/assets/stylesheets/framework/buttons.scss

@@ -371,11 +371,15 @@
 }
 
 .btn-loading {
-  display: flex;
-  align-items: center;
+  &:not(.disabled) {
+    .fa,
+    .spinner {
+      display: none;
+    }
+  }
 
-  &:not(.disabled) .spinner {
-    display: none;
+  .fa {
+    margin-right: 5px;
   }
 
   .spinner {

app/views/dashboard/todos/_todo.html.haml

@@ -48,14 +48,14 @@
 
   - if todo.pending?
     .todo-actions
-      = link_to dashboard_todo_path(todo), method: :delete, class: 'btn btn-loading js-done-todo', data: { href: dashboard_todo_path(todo) } do
+      = link_to dashboard_todo_path(todo), method: :delete, class: 'btn btn-loading d-flex align-items-center js-done-todo', data: { href: dashboard_todo_path(todo) } do
         Done
-        %i.spinner
-      = link_to restore_dashboard_todo_path(todo), method: :patch, class: 'btn btn-loading js-undo-todo hidden', data: { href: restore_dashboard_todo_path(todo) } do
+        %span.spinner
+      = link_to restore_dashboard_todo_path(todo), method: :patch, class: 'btn btn-loading d-flex align-items-center js-undo-todo hidden', data: { href: restore_dashboard_todo_path(todo) } do
         Undo
-        %i.spinner
+        %span.spinner
   - else
     .todo-actions
-      = link_to restore_dashboard_todo_path(todo), method: :patch, class: 'btn btn-loading js-add-todo', data: { href: restore_dashboard_todo_path(todo) } do
+      = link_to restore_dashboard_todo_path(todo), method: :patch, class: 'btn btn-loading d-flex align-items-center js-add-todo', data: { href: restore_dashboard_todo_path(todo) } do
         Add a To Do
-        %i.spinner
+        %span.spinner

app/views/dashboard/todos/index.html.haml

@@ -26,12 +26,12 @@
     .nav-controls
       - if @todos.any?(&:pending?)
         .append-right-default
-          = link_to destroy_all_dashboard_todos_path(todos_filter_params), class: 'btn btn-loading js-todos-mark-all', method: :delete, data: { href: destroy_all_dashboard_todos_path(todos_filter_params) } do
+          = link_to destroy_all_dashboard_todos_path(todos_filter_params), class: 'btn btn-loading d-flex align-items-center js-todos-mark-all', method: :delete, data: { href: destroy_all_dashboard_todos_path(todos_filter_params) } do
             Mark all as done
-            %i.spinner
-          = link_to bulk_restore_dashboard_todos_path, class: 'btn btn-loading js-todos-undo-all hidden', method: :patch , data: { href: bulk_restore_dashboard_todos_path(todos_filter_params) } do
+            %span.spinner
+          = link_to bulk_restore_dashboard_todos_path, class: 'btn btn-loading d-flex align-items-center js-todos-undo-all hidden', method: :patch , data: { href: bulk_restore_dashboard_todos_path(todos_filter_params) } do
             Undo mark all as done
-            %i.spinner
+            %span.spinner
 
   .todos-filters
     .issues-details-filters.row-content-block.second-block