I found a solution not very clean but its working !
As your css/js file arent compiled but minified, copy the minified file in a online converter scss/css online. Convert this minified css file into a regular scss file. You now own a full scss file containing the all css/js content of your application.
Then in your new fresh download folder "_dev" remove all the folder concerning scss and js. Erase all the content from the themes.scss and replace it with your unminified scss and thats it.
Now create again css and js folder and this is good you have access of all of your previous content which has been turned into your new themes.scss and you can add new scss and js content