Apparently there’s no official way to do this for now (CS6 , 12.0). Unofficially, you can follow these steps:
- Create a new fluid grid layout (from file > new) with your desired settings. This is temporary.
- Copy the .gridcontainer classes from the layouts (mobile, tablet & desktop) of this new fluid grid (only the ones that you changed) and paste them into the corresponding place in your main fluid grid. .gridcontainer classes are located in the css file containing your fluid grid settings.
- In the mentioned css file there’s a comment section at the top beginning with “Dreamweaver Fluid Grid Properties”. Replace new values there.
- Turn off visual aids for fluid grid & turn it on again for new changes to appear (this is done by using the button located in the Document toolbar with an eye & dotted square on it)
- This last step is a bit tricky. Probably your previous divs have lost their interactivity now & you cannot change their size or margin in the design view. This is because their widths need a bit of the adjustment. An easy solution is going to the temporary fluid grid you built in the first step (which now should look the same as your main one, in terms of fluid grid visual aids), creating a fluid div with your intended width & finally copying its width value from the css into your main fluid grid.
- Now you can delete the temporary fluid grid layout. It’s no longer needed.