After our long comment thread I thought it would be better to make a quick how-to in a PDF. Hope this solves your issue. We are working on the how-to's as we speak so that should be finished quickly.
If this still doesn't work it could be that it has something to do with Windows 10 and Scout....we use Windows 7.
I finally got it to work correctly.
I later tried setting the regional settings back to Dutch and it still worked.
I'm afraid this all might boil down to a decimal point character...
Edit: According to the reply on the issue Erik opened, there is a new release in planning. There is no indication at all when this will be available. As they seem to overhaul Scout with other runtime support it may very well take months to get a stable release. As a whole, the GitHub repo seems to be a pile up of issues without any recent fixes. Seems a dead end to me.
I really hope one of the alternatives with a more active community or other power 'behind' it will work too. I'm more than happy to put some time in helping to get it to work.
I'm sure that does not apply to everyone but if a paid alternative does work and is actively being developed, it might be well worth the investment for us.
Edit 2: The new versions of the DX themes have a new folder structure.
I now use Koala (open source, http://koala-app.com/), which works fine on win10 in Dutch . The general idea is the same, but Koala does not use input and output folders. It automatically recognizes that custom.scss must be compiled to custom.css in the css folder tree, same for lib.scss.
I just created an how-to on how to setup the Mendix UI Framework with Koala. It's really easy to use and should run automatically.
I haven't seen this issue yet during my testing. But I am going to try this out today and see if I can replicate the same issue. It sounds like you maybe missing the lib css, I think you need to map both the custom.scss folder and the lib one.