Try the below CSS in Script Editor Web Part
.ms-core-listMenu-verticalBox > .ms-core-listMenu-root > LI > .ms-core-listMenu-item
By the way, the Content/Script Editor Web Part is not a solution to apply your CSS on all quick launch headers for all pages. It will be applied only on the current page that holds this Content/Script Editor Web Part.
So in case, you have
Publishing site you can easily attach your CSS to your master page.
- Site Setting > Look and Feel > Master Page.
Otherwise, you will need to edit your master page to add the CSS to be applied on all quick launch headers site.