Single post in Site Skins: How-To, and Helpful CSS

Sorry if this question is obvious, but I did not see it anywhere (fully aware I probably missed it). If I need to change the CSS, do I need to make a new skin? I try to edit & apply the changes, and clear my cache, but nothing has worked thus far. I wouldn't be surprised if it's something I'm doing wrong, I just want to make sure that it is possible to update a skin and apply the new changes before I keep trying. The reason for this is I've found that about half of my custom field backgrounds don't work and I'd prefer not to make a new skin every time I need to test a new field background, so if someone knows the specifics on that as well (ie. if it's probably the file size, image size, size ratio, or just any idea of what could be wrong), it'd also be a great help.