So...if you have a site with 2 UI.X themes, one light one dark on XF 2.2...How would you upgrading to the XF2.3 version with variation support?
Depending on how old you're upgrading from, you might be better off re-creating your customizations in a new child style.
However, in most cases, you should be able to do the following:
Copy your light child style (where you did all your customizations - hopefully you did this

) to be a child of the Variations style, and enable Variations on the new child.
This will at least give you a solid baseline to work from.
Next, you'll want to manually copy over your color customizations put into your dark style into the dark color variations of your new child style. In case you weren't aware, an easy way to see what settings you've changed in a style is to go to Appearance -> Styles, click the 3 dot menu next to your dark child style, and select "Customized Components" (I am ashamed that it took me
way too long to realize that was there when I first got started in XF theme dev).