I want to change loading icon that appears when I click dicussion or back button.
You're probably looking for the LoadingIndicator component:
LoadingIndicator
https://github.com/flarum/core/blob/master/js/src/common/components/LoadingIndicator.js https://github.com/flarum/core/blob/master/less/common/LoadingIndicator.less
It has a class of .LoadingIndicator and inside of it there's a spinner managed by the spin.js library.
.LoadingIndicator
spin.js
You can probably switch the image with CSS, or if you want to use other options from the spin.js library you will need to extend the component with an extension.
