This simply is not possible. The UIs coding and Id changes in the XML between 8.5 and 9.0 are too much, the best way to have 8.5 and 9.0 compatible skin is to code both versions at the same time as Sunshine and I did for our skins.
Even then it's very time consuming, it takes weeks/months to make an effective skin. and it's very easy to fall into a pithole on something and get stuck for days/weeks too, as I currently am on my new skin.
/ 
