Assigning a specific stylesheet to a new side menu created


I was able to create a new left navigation side menu, with flyouts.
However, I can’t seem to apply a style to it. It is on a locked
template, which I have edited under>settings>advanced
options>page-specific stylesheets…
But, it will not apply to the left navigation side menu.
Here is the menu:

and how the menu normally looks:

Thank you.