AbigailII: 1) If you use IE it has this feature as well (never tested it though, but under Tools -> Internet Options -> Accessibility there is an option to specify your own stylesheet - but I don't know exactly how it works). Other browsers may have it too, I just can't know them all...
2) These features are usually incremental - that means you may build a CSS only changing some things. The original CSS will still be used for everything else. So, if you just specify the background color on a personal CSS, the other changes will still be reflected on your pages. Only if Fencer changes the background you won't realize because your own personal setting is overriding the server's CSS. I hope I explained myself properly...