It is a branch in git, just try it, it is called layout_rework. It works nice in the larger resolutions, but I have hit some quirks with Firefox, yes. Also, the small portrait mode resolutions on mobiles and my _javascript_ skills conflicted. I stopped when I dared to approach the minefield of Calendar, which still is so damn hardcoded html-in-C that it is a pain. We need template functions to call a week view, a day view, a tiny month view, etc etc.
I guess the rest is merely just a exercise in tightening the html pages.
The amazium (pure css) framework I use for the layout_rework branch does fit for 98% of our current way of doing things, but we would need to rethink some other parts to better fit into a grid idea. There are problems with the #content div and how it is created from C code in some cases.
Give the current branch a quick glance, if you absolutely do not like what you see, I need to start over anyway.
