How to add Navbar styling to ThemeMagic?

Could someone explain how to add the navbar colors (main background, active link, etc) to ThemeMagic? I've watched the video and know I need to add the fieldset to templateDetails.xml but not 100% sure on what exactly to add (which is the parameter I want to change?). I'm also not sure if I should be editing the main variables.less file in the less folder or if I should be editing the variables.less (or variables-custom.less) file in my theme folder.