Changing sprites


I'm learning CSS and I'm trying to modify a Gmail Userstyle sheet.
I know how to change background colors and stuff (even though most of the time I get lost with the long class names...) but I don't know how to change the icons properly.
I saw they were combined into a sprite but I don't know how I could change that file. Should I create my own sprite on Illustrator, save it as a PNG file and then upload it to any image server?

Thanks in advance for your help :)
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