Hello,
Have you created a child-theme yet? Using a child-theme you can import the parent-theme’s CSS stylesheet, then add your own style modifications after that. The good thing about a child-theme is any changes made will not be overwritten if the parent theme is updated. Additionally, this may be where you’ll want to add your styles if I have the correct element you’re looking to change.
header#siteheader .sf-menu > li a {
}
Let me know if this helps.