First make sure you copy a skin and rename the folder, don't modify existing skins or when you upgrade it might get overwritten and lose your customization. Avoid this by putting it in your own folder with a new name.
Note that skins are located in 2 places, make sure you are editing the ones under you site specific folder /Data/Sites/[SiteID]/skins
I recommend if you want a horizontal menu start with a skin that already has that, colors are the easiest things to change, changing layout is a lot more trial and error work.
ASP.NET caches the master page layout.master, web browsers cache css files.
Hope it helps,