Be careful, as I was afraid of, you are starting to make an "ABMaterial Demo App' builder. Making such a tool demands an extensive knowledge of the library and requires you having manually written a lot of webapps with the library yourself before attempting this. You should study it for many hundreds of hours. E.g. I wrote many webapps in css, html, javascript, php and java manually until I knew the languages by hart before I wrote the very first line on ABMaterial.
An example of what is happening shows already in your ABMCard. You only can add two buttons, while this is not a library limit. Be aware that the demo is just what it says: a demo. It only shows a fraction of what ABMaterial can do. Once you get to the more advanced Components like GoogleMaps or Charts for example, and the programmer can only do what is in the demo, he will be missing out on a lot ABMaterial can do. And stuff will change. ABMaterial is a living library. Things like the button.size change will happen to other components as well if they are needed. Be prepared to adept.
As for the visible grid, take in mind you'll need to do this for the three type devices: phone, tablet, desktop. Otherwise you will only take advantage of 33% of the capabilities of ABMaterial.
I don't want to discourage you, but I don't want you to be dissapointed either when you are half-way and reach the hard stuff. I do like it you make this open source in B4J! As I told some other forum members before who wanted to write such a tool, the simple components like a single button are very easy, but take into account even then, the user may want to add them as an Array component for example. Then the setup is quite different.
After I finished the Basic4JavaScript engine in ABMaterial (which will take many months), I may attempt to write a designer myself as I had some ideas on a tool myself.
I don't know if you've seen my proof of concept:
https://www.b4x.com/android/forum/threads/abmaterial-grid-builder-proof-of-concept.64721/ .Please do not recreate mine. Think of something yourself. It may be better

.
My steps will be:
1. Grid Builder for the ABMPage + Footer
2. Grid Builder for the ABMContainer (normal and collapsable)
3. GridBuilder for the ABMModalSheet
Once this framework is set for all three type devices, I'll write the stuff for the other helpers: Table, TableMutable, FlexWall etc... After that a NavigationBar builder. Only when all this stuff is working properly, I may start on the other components. This is almost the same order I used when I wrote the library. The foundation of the grid is very important if you want to succeed. The thing people struggle with the most in the beginning when they discover ABMaterial is just that. I have a feeling you do not really grasp the grid concept yourself yet, and this may give you a hard time later on. The grid is the most important part of what ABMaterial is: a responsive web framework.
What I do like very much in your tool is the 'Theme' builder! This is a very abstract concept and most people do not want (or have the time) to create their own theme so all the apps end up looking the same. If this could be worked out (maybe even with a preview?), this could be a powerful asset!
As I said, I don't want to discourage you, but please keep my advice in mind if you do not want to hit a wall further on.
Good luck!