Modify colour scheme

I've created a child theme and modified the css but I can't change the color of the menu bar. The color of the menu bar appears to be fixed by the color scheme chosen in the theme options and this seems to over-ride anything that I add to style.css. For the particular color-scheme setting that I've chosen, I've tried making changes to the relevant css file in 'Colors' on the server but these are also over-ridden. Could you tell me please if there's another step that I'm missing?