WebDirect: classic layout vs theme - different handling of layout part widths?

[xochi]

In FMS13v5, I'm adapting an old IWP layout (in classic theme) to WebDirect.   Some layouts render in the web browser with the layout part (e.g. header, body...) stretching to fill the entire width of the Browser window.  Some layouts, however, seem to size themselves only to the width of the layout set when editing the layout in FileMaker Pro.

 

I'm not sure I understand the difference, but it appears to be that Classic mode = full width, whereas if I take a Classic theme layout and update it to Custom theme, then the width = fixed width.

 

For my use, I actually prefer the full-width look.

 

Is there any way to have a new-style theme but keep the full-width behavior?