Kodi skins only change the way the software looks and what information is displayed. As such, installing skins should never cause you to lose any addons.
That said, you might lose them if you inadvertently install a build instead of a standalone skin. That’s because builds are effectively heavily-modded Kodi installations that come with their own look and bundle of (usually piracy-focused) addons. We advise against using builds but have no issue with custom Kodi skins on their own.