How can I add some style to title of side bar block? I want to add some style to my drupal 8 Sidebar menu, I want to create something like below image with Font awesome and css:
First add position: relative as a style to the containing element (most likely the nav element.
Add a pseudo element after the h2 element, which you probably want to have your specificity include the class of the nav element.
Style the pseudo element with position: absolute; right: 0;
Tweak the styles of the h2 and h2:after blocks to suit your needs.