Fallback theme for 18.04

I have a theme which does not work on Inkscape snap. But I like that theme and would like to use it as Inkscape is the only singularity. Is there a method by which we can specify a fallback theme ( say Adwaita ) in index.theme or somewhere else ? So that I can use my theme and where it does not work, it will use Adwaita. Thanks.

Using Ubuntu 18.04.