Hi Juan,
you can add the CSS/JS reference in the MANIFEST of the WebLayout
CSS-DesignLibs: 12grid/lib/12grid_design.css
CSS-ClientLibs: 12grid/lib/12grid.css
(For javascript JS-DesignLibs/JS-ClientLibs )
Note you can have a CSS-DesignLibs which differ from CSS-ClientLibs. The design libs is used in the Form Designer, while client lib in the running client.
The reason for this is that you can add custom style to your layout so it can be better used in the Form Editor; for example we give a min height of 20px to all 12grid layout elements so the user can drop easily elements into it using the form editor.
Check our 12Grid layout
https://github.com/Servoy/12grid/blob/m ... ANIFEST.MF