Hello there.

Few weeks ago i switch from ElvUI back to Standard WoW UI. For some reasons my BigWigs (setting window) still has the ElvUI skin. I only installed elvui and addonskins. (Now i deleted both)

In BigWigs i cant change the skin. And ElvUI discord told me, that the problem doesnt come from ElvUI.

Maybe someone of u guys can help me to fix this?

If you do not have addonskins and elvui in the addon folder, restarted the wow client, ensure they do not show in the addon list in game to verify they are no longer installed.... then your issue wouldn't be with elvui or addonskins as the addons can not effect anything since they are removed...
TLDR; if you do not have elvui or addonskins installed and not in the addon folder and isn't shown as installed and running then neither addon is modifying anything.