ElvUI doesn’t skin the blizzard elements. It has its own modules.
ElvUI is a complete UI replacement.
If you only want the Unitframes, it would be far better to download a dedicated Unitframe addon.
You can go through and disable each ElvUI module. See the ElvUI menu and go to each section and uncheck enable. I advise just to get a separate Unitframe addon.
I always recommend backing up your WTF folder - where your profiles/settings for all addons, including ElvUI are stored.