How To: Black on White Menu Bar, no gradient

I would like to change the menu bar across the top of my site to black on white with no gradient. I added 'white' in default-options.php and created a white.css, chose 'White' and published, but still got a white on gray-gradient for the menu. I imagine that a sub-theme is the right way to accomplish this, but I am new to theme customization and have not yet figured that out? Is there an easy way to accomplish this? Is there a guide to sub-themes and a tip on how to customize that? https://blog.ioncomputer.com/