But i'm missing the design part of it, a place in the app where you can customize or make a new theme for your HTML.
Could be something like (Which im using for now): https://pikock.github.io/bootstrap-magic/app/index.html#!/editor
The problem about this is that if i have to change something, i have to go to that website and upload and less file, make changes and then again import it into Boostrap Studio. Could be a great feature in app.
As Frank mentioned, we are planning for SASS support in Bootstrap Studio in the next releases. This will make it possible to not only write SASS code in the app, but to customize Bootstrap as well. We will show the bootstrap.css file in the Stylesheets group, and right clicking it and choosing "Properties" will open up a dialog with all the options you see in that great tool you linked. When you save your changes, Bootstrap.min.css will be recompiled and your design will update to show your changes.