You might want to try using the :before selector. I use it in a variety of places in my theme such as topic/week titles, blocks, course titles, etc. I got the idea from the Ardvark theme and used some of that code in the styling of blocks:
Here is an example for various blocks.
https://github.com/kennibc/moodle-theme_fordson/blob/master/scss/preset/default.scss#L353-L559