I suppose you want to later make use of the theme accent color in the default theme? I tried doing something along those lines a little while ago. It's here if you would like to take a look at it for inspiration. I stopped because, with CSS variables, I didn't have access to all the SCSS functions like darken, etc. This made it a very tedious process to convert it to base CSS. Maybe there is a better solution?
I suppose you want to later make use of the theme accent color in the default theme? I tried doing something along those lines a little while ago. It's here if you would like to take a look at it for inspiration. I stopped because, with CSS variables, I didn't have access to all the SCSS functions like darken, etc. This made it a very tedious process to convert it to base CSS. Maybe there is a better solution?